If you see a BOOTMGR is Missing error or similar message early in the boot process, you have a BCD problem. This article explains how to rebuild the BCD.

Although it looks difficult, the whole process should only take about 15 minutes. Here’s what you need to do:

If you're using Windows 11, 10, or 8, open the Advanced Boot Options menu, and then select Troubleshoot. If you're using Windows 7 or Windows Vista, open System Recovery Options.

Select Advanced options in Windows 11/10/8.