Check your optical drives, USB ports, and floppy drives for media. Often times, the "BOOTMGR is Missing" error will appear if your PC is trying to boot to a non-bootable disc, external drive, or floppy disk.
Note: If you find that this is the cause of your issue and it's happening regularly, you might want to consider changing the boot order in BIOS so the hard drive is listed as the first boot device.
Perform a Startup Repair of Windows. This type of installation should replace any missing or corrupt files, including BOOTMGR.
Even though a Startup Repair is a common solution for BOOTMGR problems, don't worry if it doesn't fix your problem. Just continue troubleshooting - something will work.