How to fix problem after deleting ubuntu? [duplicate]

I have Windows 8 pre-installed, and then I installed GRUB with Ubuntu. Ubuntu is not my thing, so now I want to remove it along with GRUB. From what I have learned, with UEFI, GRUB does not overwrite the windows bootloader in the EFI partition and is stored elsewhere. How would I remove GRUB and make my PC use the Windows bootloader instead? It should be noted that I created a separate /boot partition when installing Ubuntu.

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)

Run a cmd.exe process with administrator privileges
Run diskpart
Type: list disk then sel disk X where X is the drive your boot files reside on
Type list vol to see all partitions (volumes) on the disk (the EFI volume will be formatted in FAT, others will be NTFS)
Select the EFI volume by typing: sel vol Y where Y is the SYSTEM volume (this is almost always the EFI partition)
For convenience, assign a drive letter by typing: assign letter=Z: where Z is a free (unused) drive letter
Type exit to leave disk part
While still in the cmd prompt, type: Z: and hit enter, where Z was the drive letter you just created.
Type dir to list directories on this mounted EFI partition
If you are in the right place, you should see a directory called EFI
Type cd EFI and then dir to list the child directories inside EFI
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).

Put the DVD in your optical drive and boot from it.
Press a key when it displays Press any key to start from CD or DVD.
Select your language etc. and click Next.
Click Repair your computer.
Click Troubleshoot.
Click Advanced Options.
Click Command Prompt.
In the command prompt window, type bootrec /fixmbr
Click the red X to close the command prompt.
Click Turn off your PC.
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:

Hit Windows+X and select Disk Management.
Find the Ubuntu partition. It will probably be a large partition without a drive letter.
Be sure you have the correct partition!
Right-click the partition and delete or reformat it with a Windows filesystem.

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

put it in your optical drive and boot from it
on the installation screen where it asks you to install windows, click on Repair Your Computer on the lower left corner of your screen
Now go to command prompt (It probably will show a window saying "Trying to repair windows automatically", close it) and type BootRec.exe /fixmbr
after it finishes GRUB is gone and you can now boot into windows directly
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"
Select the Ubuntu partition and format it to a file system that windows can use.

Remove Boot Loader EFI / System Partion in Windows 10

Quick Guide

Open PowerShell as Administrator

mountvol S: /S
S:
cd .\EFI\
dir
rd /S Ubuntu

Details

Administrator PowerShell

Enter powershell into the windows search of the start menu. Look for a blue icon with the label "Windows PowerShell", right click on the that and select "Run as Administrator" within the context menu.

Mount EFI/System Partition As Volume

To Mount the EFI System Partition on the given drive use the mountvol command by using the /S parameter. You can chose any free drive letter you want. For example "S".

mountvol S: /S

Access Mounted Volume

Now the partition is mounted. We can access the mounted volume by changing to the drive by the cd command and the drive letter S: as parameter

cd S:

To ensure we are at the root of this volume we should execute:

cd\

Type via the dir command to list the entries of the current path to ensure you on the right one.

dir

It should look like this:

Directory: S:\


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
d-----       2019-01-17     12:55                EFI
-a----       2018-10-16     10:57             31 startup.nsh

Delete Boot Loaders

Your boot loaders are location in the EFI directory. Change to it by using the cd command and list the entries via the dir command.

cd .\EFI\
dir

Your output depends on the boot loaders you have installed, here is a example with Windows and Ubuntu.

Directory: S:\EFI


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
d-----       2018-12-06     06:55                Ubuntu
d-----       2018-12-05     05:21                Microsoft
d-----       2019-01-17     12:55                Boot

Now you can delete unwanted loaders via the command Remove-Item and the parameter -Recurse.

DO NOT DELETE THE BOOT DIRECTORY OR THE WINDOWS DIRECTORY!

Remove-Item -Recurse .\ubuntu