Compaq 900Z keyboard photo

If you have a regular computer monitor available, try plugging that into the laptop's VGA socket and follow the instructions inn the laptop manual for switching to an external display. If the image appears correctly on the external monitor the problem is probably with the laptop screen or the connector between it and laptop's main board.

Another troubleshooting hint is to shine a torch on the laptop screen when the laptop is running -- if you can see something the screen's illumination has failed.

Either way you need to see a repair guy -- or if the laptop is less than five years old take it back to the dealer and ask why the thing has failed.