Hi,
Perhaps there was a bios/supervisor password created. That needs to be entered on the blank screen.
If you don't remember the bios/supervisor password the try the following:
Unplug the PC
Remove the back cover
Remove the motherboard EMI shield
Remove the battery (round coin object) and wait 30 seconds
Replace the battery
Replace the EMI shield
Replace the back cover
Plug in the PC and power on.
Your PC is still under warranty so HP should be able to further assist you with your issue.
HP's support number is 800-474-6836 or 800-HPINVENT or online here.