Man you don't know how much you saved me thanks souch I don't know anything about PC's I had an error saying "you have no bootable devices" and you fixed it thanks heaps man you definitely deserve more subs❤
For your issue, the system can not detect the OS, re-installation of the OS would be the last resort. If it happens again, you may check first the boot options or boot priority in your bios choose the drive that has the OS. Some of the cases happen when the system reset or cmos malfunction the bios setting resets.