Please do try these troubleshooting steps that I just suggested to someone who had the same problem.
- Start with a hard reboot, unplug it's AC adapter then press and hold the power button for 20 seconds then try booting it.
- If it doesn't work reseat the RAM, remove them all for couple of second then put them back in making sure everything is seated properly.
- Again if these doesn't work but you can hear the laptop on but with no display, connect an external monitor and see if it would display anything because if it does that means it's the built in display that is faulty.
- Next to try is if your AC adapter is still okay by trying another AC adapter that has the same voltage and see if it would work.
- If all these will not work then the problem is with the motherboard.
- Do contact HP if it's still under the factory warranty.