How to Fix Windows Boot Manager and MBR in Windows 10

To repair the Windows Boot Manager and MBR, follow these steps:

Using the Startup Repair Tool

Start your computer from the Windows 10 bootable media.
On the “Install Windows” screen, select “Repair your computer”.
Choose “Troubleshoot” from the “Choose an option” screen.
Select “Startup Repair” from the “Advanced options” screen.
After the repair process is complete, select “Shutdown” and then turn on your PC to see if Windows can boot properly.

Repairing Boot Codes using Command Prompt

Start your computer from the Windows 10 bootable media.
On the “Install Windows” screen, select “Repair your computer”.
Choose “Troubleshoot” from the “Choose an option” screen.
Select “Command Prompt” from the “Advanced options” screen.
In the Command Prompt, run the following commands:
bootrec /fixmbr
bootrec /fixboot
Restart your computer to check if the problem is resolved.

Fixing BCD Errors

In the Command Prompt, run the following commands:
bootrec /scanos
bcdedit /export c:\bcdbackup
attrib c:\boot\bcd -r -s -h
ren c:\boot\bcd bcd.old
bootrec /rebuildbcd
Restart your computer.

Replacing the Bootmgr File

If the above methods don’t fix the problem, you can try replacing the Bootmgr file:
In the Command Prompt, change the directory to the System Reserved partition.
Run the attrib command to unhide the file: attrib -r -s -h
Navigate to the system drive and run the same command: attrib -r -s -h
Rename the bootmgr file as bootmgr.old: ren c:\bootmgr bootmgr.old
Navigate to the system drive.
Copy the bootmgr file, and then paste it to the System Reserved partition.
Restart your computer.