The No Signal Sign Apears.

Hello,
This is my very first time using this site and hope you could help me. My problem is that when I switch my computer on it comes on perfectly. The light are all on, the wires are all plugged in etc and it seems to be working fine. However, when I switch the screen on, a sign saying 'no signal' comes up and nothing else seems to happen. I have bee trying very very hard to fix it so I would be very releived and thankful if you could come up with a solution as soon as possible!!!!!! Thank You.