turn the lights off in the room and see if you can see light coming through the vents on the back of the tv when it is on. You could even use a torch shining on the screen to see if there is a picture there. If there is no light coming from the vents in the back, and you can just see a picture when you shine the torch ligh on the screen, then the panel back light has stopped working. Depending on how old the tv is, there is little you can do about it unfortunately. If it is in warranty, then all good. There is a way to repair these, but only if you are up to the job!