Update: I was able to boot up after removing and re-seating CMOS battery. I was even able to update several programs and do several reboots. My HD was still "dangling" inside the tower, so I had to turn it off first, then fix it in its place. After placing the HD back in its place, I was back at the same problem again.
I removed and reinserted CMOS battery once more, in hope that it would fix the issue, but still...
(*always checked the BIOS settings after each removal and replacing of CMOS battery)
I do not know what to do anymore.