The solution to a damaged partition boot sector is to overwrite it with a new, properly configured one using the bootrec command, a relatively simple process that anyone can do. These instructions apply to Windows 11, Windows 10, Windows 8, Windows 7, and Windows Vista.
Windows Boot Partition Recovery | Including Files and Partitioning
If the partition boot sector becomes damaged or misconfigured in some way, Windows will not be able to boot properly, resulting in an error message such as "BOOTMGR is missing" very early in the boot process. When this happens, follow these steps:
Launch Advanced Startup Options (Windows 11, 10, and 8) or System Recovery Options (Windows 7 and Vista).
At the prompt, type the bootrec command as shown below and then press Enter.
