techies of ttg I need your help.

for the last few weeks whenever I download a game on steam (this seems to be the main trigger, I don't know why) midway through the download my pc will crash, and when I attempt to boot it up again it comes up with the BIOS menu saying it can't find anything to launch. One out of maybe 20 attempts you will be able to get in, but usually you will then crash again after a few minutes max. When the download ends the problems seem to cease.

does anyone know why this may be happening? I worry that if I don't resolve it soon eventually I will be locked out and it will be too late to do anything.