some hp especially the dv series laptops have the problem where over time the gpu basically starts unsoldering due to heat which model hp is it--or may be the inverter in which case if you look really closely at the screen you should just be able to make out the desktop although the screen looks black--or else it could be the screen ribbon over time with all the opening and closing of the lid a break can occur in the ribbon---try to connect an external monitor to rule out possibilities--if you get display on a monitor its not gpu
could also be the ccfl bulb in the lcd though thats less common