It seems trivial, but sometimes unplugging the computer (even from the UPS, if any) for a few seconds will resolve an issue where the computer does not properly boot, or even load the system BIOS. Plug it back in and see if the HP logo/BIOS screen appear when you turn it back on. If so, but Windows doesn't load, it's an update issue. If nothing appears it's most likely a hardware issue.

John