Is your laptop's screen pixels broken up like a bad signal? You shut off and tried to restart that laptop and screen is green with pixels looking messed up like a bad TV reception on satellite TV. You can hardly see desktop icons or wallpaper.
You can try powering off that laptop and reseating a battery. Power it back on. You can also try to use that laptop without a battery and just an AC Adapter.
If there is no display with that laptop's LCD and when you hook up to an external monitor to a VGA connection, then you might want to make sure you didn't accidentally turn down LCD brightness. Make sure you didn't accidentally hit a function key that would turn off that display entirely or put that laptop into a different mode ie. a sleep mode. Does that screen look similar or same when hooking it up to an external monitor?
If you didn't accidentally change any LCD settings then it might be a hardware issue with that laptops video card. If your computer is fairly new, HP/Compaq should repair or replace it with their depot service. It sounds like to me either that LCD screen, LCD inverter board, or Video Card is malfunctioning.
I usually hook up laptops directly to an external monitor to troubleshoot quickest. Make sure when hooking up to an external monitor to use a FN function key with correct key on that laptop keyboard to make a video display show up on your monitor and not that laptops LCD. If that picture on that external monitor looks normal then it is a problem with your laptop LCD.
If a picture on that external monitor is terrible or non existant then it is an issue with your video card or video drivers. You can check out this notebook display problems page from Compaq. Ok great you can at least use an external monitor for now.
That laptop video card is fine then. Can you follow some troubleshooting steps from following laptop repair help article. Your laptops video card is fine so just worry about those steps about your laptop screen.
If it resembles any of sections where that LCD screen needed replacement, then I'm afraid you need that lcd screen and or inverter replaced on your laptop. At least it works on an external monitor for now. These tips were written for a Compaq Presario C762NR Notebook PC, but can certainly be used as a basis to troubleshoot laptop LCD problems on other brands and model notebooks and laptop computers.