To determine if your back lights have stopped functioning do the following: Turn tv on and using a flash light or other light source, shine light directly into tv screen cup hands around your eyes and get right up to the screen and view into the screen, look very closely to see if you can make out an image, it will be faint. Try accessing the menu or changing the channels or something that would cause some graphics on the screen. If you can see an image then this indicates the back lights are not functioning, probably due to a faulty inverter board. Please let me know your results.
Your tv must have suffered an internal electronic failure. My suggestion would be to contact an authorized service center and see if you can get an estimate of repair. I can not tell you exactly what is wrong, due to too many variables that are not foreseeable from the internet. If I suggested a particular failure it would be just a guess. Sorry. Let me know if I can assist you further. Thank you