On the MB there is a reset BIOS jumper. Your MB manual will describe how to use it. Power off first and unplug unless the MB manual says otherwise.
If you are seeing a BIOS message telling you to reboot then you may have already POST'ed. Check and see if the hard drive is spinning up (you can hear it usually, if not then press a wooden chopstick against the drive cases and listen to that). Download a bootable diagnostic from the HD maker (either burn CD or build bootable USB, the HD maker (e.g. seagate, western digital, etc.) will have instructions. Sometimes a PSU takes parts like HDD with it when it fails.
If your disk is good and the PC is booting then create a windows diagnostic disk (you can create one from the same level of windows as you run). It might be asd simple as a broken master boot record.