For starters, check to make sure the brightness setting is high enough so the screen can be seen. This can be done by holding the Fn key, and pressing F7. If this doesn't work, check that your computer isn't set to be used with an external monitor. To do this, hold the Fn key, and then press F10 to change the current display setting. This will cycle through 3 different display settings, so you may need to press it more than once. If this also doesn't work, you may need to consider replacing the display on your computer.