It's relatively quick and easy to fix if you have a live boot Linux usb stick ...and probably a second machine so you can Google what to do. It's just also rather worrying at the time.

My old thinkpads have this great feature where the hard drive is easilly accessible on the side, so I leave the cover off and just swap the drive to boot into a different os

iirc the last time it happened to me, i just needed to fix the uefi entry which wasnt that bad.
(just remember to have a usb stick with a live image ready)

if it were to overwrite your bootloader that would be a way harder fix.

i dont remember if the second ever happend to me