Unplug the system from wall power, hold in the power button for about 10 seconds, pull the CMOS battery for about 10 seconds to reset the BIOS, try again.
Providing some more details is a good idea, the computer model, what happened before the issues started, if it just happened out of no-where, does it not turn on at all or does it try to start but shuts down or reboots? Need full details. "HP Desktop PC" runs the range of many years and many many different computers. You could have a 15 year old system or a brand new $3,000 workstation.