Mystic (nprofile…zpcw) I've had PC's lose the assigned boot drive in the BIOS, with no drive detected in the BIOS, after a Windows update.
As 🐘🐘 Humpleupagus 🐘🐘 (nprofile…nxac) said, a dead CMOS battery could cause this.
Worth a check to see if the disk is actually detected in the BIOS. If it's not seen, try to manually select the disk in the boot menu area.