How to fix "BOOTMGR is Missing Errors"...
1. Restart the computer. Sometimes this is all you need to do.
2. Check your drives (CD/DVD/USB) and make sure nothing is in them. Often the error shows up if your computer is trying to boot from another device, disk, etc.
3. If this is happening regularly, you might want to look into changing the boot order in BIOS, so that your hard drive is listed as the first boot device (if it isn't already).
4. Perform a Startup Repair of Windows. Doing this should replace any missing or corrupt files, including BOOTMGR.
5. Write a new partition boot sector to your Windows system partition. This may correct any possible corruption, configuration problem, or other damage.
6. Rebuild the Boot...