PC is asking for a bitlocker recovery key i've never seen

Don't reinstall anything yet, the key almost certainly exists somewhere. A lot of laptops switch on device encryption by themselves the first time you sign into Windows with a Microsoft account, and the recovery key gets backed up to that account without telling you. On your phone or another computer, search for microsoft recovery key, sign in with the same Microsoft account you use on the PC, and look for the device in the list. Match the Key ID shown on the blue screen to the one on the page, then type in the 48 digit number. If it's a work or school PC, your IT people have it, not you. It usually gets triggered by a BIOS/firmware update or a change to boot settings, so if this keeps happening after you get in, that's worth looking at.