This article provides solutions to BOOTMGR errors.
bootmgr is missing press ctrl+alt+del to reboot how to fix || Change boot menu for this problem
Restart the computer. The BOOTMGR error may be a fluke.
Check your optical drives, USB ports, and floppy drives for media. Often, the error message "BOOTMGR is Missing" will appear if your PC tries to boot from a non-bootable disk, external drive, or floppy disk.
Check the boot order in the BIOS and make sure the correct hard drive or other bootable device is listed first, assuming you have more than one drive. If the wrong drive is listed first, you may see BOOTMGR errors.