hp pavilion won't power on at all, no lights

Do a proper hard reset. Unplug the charger, hold the power button for a full minute, plug only the charger back in and try again. Sometimes the controller gets stuck after the battery runs completely flat. If nothing, try the HP recovery combo: with the charger plugged in, hold the Windows key and B together, then press power for 2 or 3 seconds and let go of power but keep holding Win+B. If the screen comes on and you hear beeps, it's recovering the BIOS, let it finish. No reaction at all to any of that at four years old, I'd suspect the charging port or the board.