I know this is an old thread but it sounds to me like poor connection to the florescent tubs that provide the back light. If it turns on for a second maybe displays the dell logo or hdmi etc connection when you first turn it on then I'd say the screen is working fine. If there is poor connection to the many back-light tube (even one poor connection will do) the board will protect itself and power off the lights. I found some of the clips had gone brittle with heat fatigue so used caption tape to reapply pressure to the contacts. I also used switch cleaner to clean the contacts and the lights now stay on.
