petri[.]com/windows-boot-manager-fix-startup-errors/
Since hardware moved to UEFI BIOS, Windows boot files [Boot Mgr., BCD etc.] can occasionally cause some real headaches. Problems *usually* occur only after moving, migrating, or installing Windows, so most people will never [or seldom anyway] have to worry. You want the boot files on a small FAT32 partition that's normally hidden -- for several versions of Win10 it was also protected, the size grew from ~100MB to often 300MB, and OEM vendors can add their own twists -- and while FAT32 may not be technically necessary, that's the Only way many BIOS will find it. If there's a problem with the boot files the free EasyBCD is the first choice to try & fix things, followed by the boot repair tools on the Macrium Reflect [free or paid] or Paragon HDM [paid] USB sticks. Microsoft has its own tools [mainly BCDBoot & BCDEdit, though there are a few more] that are included with Windows, but they're not easy and in the past often have not worked. Nonetheless, if you get into a bind one of the methods in this article might help.