It is probably on the wrong auxillary* or input. Try finding the remote and on it it will say "Input". Depending on your type of cable/box, try switching it around to see if you can get a picture. I would personally try the tv or cable box input if there's one. If you have no remote, on the tv, there should be another choice for input.
If none of this still works...
-Try re-plugging all the wires in the back and then plug them back in to their rightful spots.
-See if the input cables (yellow, white, and red) are hooked up to the right color spots, or see if there is a cable loose in its jack)
-Unscrew/rescrew cable wire
-LASTLY, which is unlikely, because you will be able to fix it...Call Expert/BUY new tv