The Windows Boot Manager (BOOTMGR) is tiny software which runs as the system starts and loads the operating system from the hard drive. But in many users get the Bootmgr missing error on Windows operating system.
Well, there are many reasons like hard drive bad sectors, bootmgr files corruption, or outdated BIOS and other. However, this can be fixed by following various fixes.
First, restart your PC/laptop; remove the entire connected media, update the BIOS and check for the hard drive issues, hope this works for you. But if not then rebuild the boot configuration data (BCD).
Hope this works for you.
To follow the complete solutions visit: https://www.fixwindowserrors.biz/blog/resolve-windows-bootmgr-is-missing