A friend of mine has a Dell Vostro 1720, and he says that when he switches it on the screen will be blank. It remains blank until he taps the back of the screen on both sides a few times. After this, the screen will come on and stay on for the rest of the day until it is switched off.
He has checked the connections on the motherboard etc and they seem to be fine. I have a feeling that it might be the actual TFT panel, but it doesn't seem to be a right diagnosis since the screen will then stay on the rest of the day.