Be sure to let me know what steps you've already taken to resolve the "BOOTMGR is missing" issue.
This page details the procedures for installing and reinstalling GRUB 2.
Failed while updating boot sectors disk
Another reason you might see BOOTMGR errors is if your PC is trying to boot from a hard drive or flash drive that is not properly configured to be booted from. This also would apply to media on an optical drive or floppy drive that you're trying to boot from.
If you're interested in fixing this BOOTMGR problem yourself, continue with the troubleshooting in the next section. for a full list of your support options, plus help with everything along the way like figuring out repair costs, getting your files off, choosing a repair service, and a whole lot more.
BOOTMGR issues apply to Windows 10, Windows 8, Windows 7, and Windows Vista operating systems only. The equivalent function in Windows XP is NTLDR which produces the NTLDR is Missing error when there's a similar problem.
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For more information on a variety of GRUB 2 topics, please visit the GRUB2 main page.
For information on converting to or from GRUB Legacy, please view the Grub2/Upgrading community doc.The GRUB 2 bootloader is included on all currently-supported versions of the Ubuntu family.The "BOOTMGR is missing" error displays shortly after the computer is turned on, immediately after the Power On Self Test (POST) is complete.Windows has only initially started to load when the BOOTMGR error message appears.There are a number of possible causes for BOOTMGR errors, including the most common "BOOTMGR is missing" error message.The most common reasons for BOOTMGR errors include corrupt and misconfigured files, hard drive and operating system upgrade issues, corrupt hard drive sectors, an outdated BIOS, and damaged or loose hard drive interface cables.