This answer is for those with UEFI who have deleted the Ubuntu partitions before removing grub

You will be doing this from Windows 10. No bootable media required.

Where bootrec /fixmbr, bootsect /nt60 and the Ubuntu live with the boot-repair suggestions have failed, this has worked for me:

(This answer borrowed verbatim from here)

1. Run a cmd.exe process with administrator privileges


2. Run diskpart
   


3. Type: list disk then sel disk X where X is the drive your boot files reside on


4. Type list vol to see all partitions (volumes) on the disk (the EFI volume will be formatted in FAT, others will be NTFS)


5. Select the EFI volume by typing: sel vol Y where Y is the SYSTEM volume (this is almost always the EFI partition)


6. For convenience, assign a drive letter by typing: assign letter=Z: where Z is a free (unused) drive letter


7. Type exit to leave disk part


8. While still in the cmd prompt, type: Z: and hit enter, where Z was the drive letter you just created.


9. Type dir to list directories on this mounted EFI partition


10. If you are in the right place, you should see a directory called EFI
    


11. Type cd EFI and then dir to list the child directories inside EFI
    


12. Type rmdir /S ubuntu to delete the ubuntu boot directory

Assuming you only ever had two operating systems (Win 10 & Ubuntu) you should now be able to boot directly to Windows without hitting the black grub screen.

To restore Win 10 default bootloader follow these steps:

• Log into Win 10


• Open Command Prompt (Admin)


• c:> bootsect /nt60 <drive name>: /mbr

<drive name> is the drive letter where the Master Boot Record (MBR) will be updated

For example to update C master boot record this is the command:

c:> bootsect /nt60 c: /mbr

For more help about bootsect command see here -
https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/bootsect-command-line-options

You can restore the Windows bootloader with a Windows 8/8.1 DVD. These instructions are inspired by Manindra Mehra's answer, but I expanded it with full working details (verified with a Windows 8.1 DVD).

1. Put the DVD in your optical drive and boot from it.




2. Press a key when it displays Press any key to start from CD or DVD.




3. Select your language etc. and click Next.




4. Click Repair your computer.




5. Click Troubleshoot.




6. Click Advanced Options.




7. Click Command Prompt.




8. In the command prompt window, type bootrec /fixmbr




9. Click the red X to close the command prompt.




10. Click Turn off your PC.




11. Turn the PC back on and it should boot directly into Windows.

This leaves the Ubuntu partition on your hard drive or SSD. To remove it:

1. Hit Windows+X and select Disk Management.




2. Find the Ubuntu partition. It will probably be a large partition without a drive letter.




3. Be sure you have the correct partition!




4. Right-click the partition and delete or reformat it with a Windows filesystem.

To do so you will need a windows installation cd/dvd

1. put it in your optical drive and boot from it


2. on the installation screen where it asks you to install windows, click on Repair Your Computer on the lower left corner of your screen


3. Now go to command prompt (It probably will show a window saying "Trying to repair windows automatically", close it) and type BootRec.exe /fixmbr
   


4. after it finishes GRUB is gone and you can now boot into windows directly


5. you have an Ubuntu partition left in your computer, that doesn't show in "My Computer", to access that, right click on "My Computer" and Select "Manage" and go to "Disk Management"


6. Select the Ubuntu partition and format it to a file system that windows can use.

With UEFI you have both a Windows folder & an Ubuntu folder in the efi partition. the UEFI reads the efi entries and adds them to its own NVRAM to remember them. You have to remove ubuntu folder from efi partition first or UEFI will re-add it. Then you have to remove UEFI entry from UEFI.

You should have these folders in the efi partition. Delete only the ubuntu folder. Live installer should show folders. And if only Windows you have to mount from inside Windows the efi partition as it is not normally mounted.

/EFI/Boot
/EFI/Microsoft
/EFI/ubuntu

You should not have to install Ubuntu but can use live installer DVD or flash drive. Some UEFI systems may let you do the UEFI edit from UEFI menu.

from liveDVD or flash and use efibootmgr

sudo efibootmgr -v

The "-v" option displays all the entries so you can confirm you're deleting the right one, and then you use the combination of "-b ####" (to specify the entry) and "-B" (to delete it). Examples #5 is delete:

http://linux.dell.com/cgi-bin/gitweb/gitweb.cgi?p=efibootmgr.git;a=blob_plain;f=README;hb=HEAD

http://software.intel.com/en-us/articles/efi-shells-and-scripting/

You can also use a USB memory stick for this job. It takes three applications: Unetbootin, FreeDOS and Testdisk for DOS.

1. First download Unetbootin and use it to make a bootable USB memory stick. Choose Freedos when prompted for a distribution to install on it.


2. Download Testdisk for DOS (don't pick the beta, but pick the stable version).


3. Unzip the files testdisk.exe and CWSDPMI.exe, and put them on the bootable memory stick (not in a folder, just straight on it).


4. Boot your computer from the memory stick. At the Default window, simply press Enter.


5. Now FreeDOS is being launched. Choose: FreeDOS Safe Mode (don't load any drivers) and Press Enter.


6. Then type: C: and Press Enter.


7. Now type: testdisk and Press Enter.


8. Select [No Log] and Press Enter.


9. Select the hard drive concerned: usually the second option (the first option is the memory stick itself) and Press Enter.


10. Select [Intel] and Press Enter


11. Select [MBR Code] and press Enter. When prompted, type y (yes) and press Enter again.

Now you're done! Reboot your computer normally. Your computer should boot up Windows now.

If All Else Fails!

The guy above me's method does not work on newer EFI computers. I solved the problem. Here is how I did it. WARNING, you have to reinstall Linux / grub first!

Steps: (this is if nothing else works and takes a long time)

1: Reinstall Ubuntu / Linux mint (this is just so you can use GRUB to boot into Windows).

1.5: Restart and boot into windows (if you can't boot to Windows, then live boot from the CD or USB and run the following in a console:

If you have a windows repair disk you can select the UEFI firmware option and load Windows from there (to avoid reinstalling Linux)

Boot repair (if needed right now)

sudo add-apt-repository ppa:yannubuntu/boot-repair

sudo apt-get update

sudo apt-get install -y boot-repair && (boot-repair &)

1g: Select recommended repair and follow the on screen instructions.

2g: After your done, reboot. You should see the grub menu, even though you can boot to Windows from here, this is not what we want yet... But find the option that boots into Windows.

2: Once your booted into Windows, run your disk partition editor and delete All partitions related to Linux / grub. Especially make sure the small grub partition is removed. Reboot

2.5: Make sure the windows loader is the first selected boot device. (most likely is). Disable the Ubuntu option. Continue boot.

3: If you've done everything right up to this point you should see (depending on your computer) your splash screen for a second and then it will turn into an error screen saying that there was an error with the boot. ("Winload.exe is missing or corrupted" most likly).

3.5: Don't be alarmed (I was). The next step will restore your original bootloader.

4: Restart and boot to your Linux CD or USB.

5: Once booted, run boot repair commands from above again. This time when running it. It will act differently, there is no grub bootloader detected to reinstall and should run faster than before.

6: Once complete, restart.

7: Enjoy your GRUB free system!

This worked for me when nothing else did, I ran bootrec.exe. Nothing worked. This saved my computers life. Hope it saves yours!

No CD's, USB's, DVD's. No long tutorials.

On UEFI all you have to do is:

Place Windows boot entry in NVRAM as first.

How to do this - several solutions.

Easiest solution is to use firmware functionality and reorder NVRAM boot entries.

What worked for me is:

1. Boot to Windows


2. Win + X


3. Command Prompt (as admin)


4. bcdedit /set bootmgr path EFIMicrosoftBootbootmgfw.efi


5. Reboot

For system with GPT partition table, the method provided by @Ganesh Kondal won't work. Using bootsect /nt60 C: /mbr will show that it worked while it really doesn't. Using bootrec /fixmbr will lead to error that "the system cannot find the file specified". If any of the above situation applies, do the following (Using Windows 10 as an example):

1. at log in prompt, hold shift, click on power button, choose restart, let go of shift


2. choose troubleshoot -> advanced options -> command line


3. at command line, type in bcdboot <drive name>:windows
   

for more detailed information, also look at this webpage:https://www.tenforums.com/general-support/74226-bootmgr-error-cant-find-fix.html