Sounds like the BIOS has been corrupted. This commonly happens after running HP's own "Health Check" and following its recommendation to update the BIOS. Things appear to work fine, but after the machine turns off, you are not able to restart it, and it exhibits the symptom you describe. It is possible to re-flash the BIOS using a Phoenix Crisis Recovery Disk in an external USB drive, but it's time-consuming to find the right versions of PHLASH16, the BIOS files, etc. I find it hard to believe that HP allows this extremely serious and well-known problem to persist, and does not provide any information on how to recover. They tell people to send the machine in to them to fix — at the users expense.