Here I try this from my bootable USB Window 10 install media. I don't use the full installer but it's repair option for failed boots.

More at:
Skip down to "Use installation media to restore or reset your PC" where you make your USB stick and boot that. I'd head to the automatic repair in this case and try that first.

Automatic repair from our W10 install/repair stick is a screen like we see at:
https://www.winhelp.us/repair-your-computer-in-windows-10.html
Skip down to "Booting to Recovery Environment with Windows 10 installation media or Recovery Drive/System Repair Disc" and there are the screens where I'd try automatic repair.

Worst case is a wipe out and reinstall. As everyone knows, backups on PCs is no longer optional.

-> I've seen this happen if the owner changes the BIOS HDD mode from something to something else so if you did that, put it back.