Try this, make sure there is video being sent to the screen, if you have sound then it should have video .
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.
If there is no image, even using an external light source, then probably the signal board has issues. It is a quite expensive board. I would suggest to contact an authorized service center and arrange for an estimate of repair. We could guess at which board is bad, but if it is not the right one it will cost just as much as having a service tech repair it the first time. Hope this helps,
If you can see images after the screen goes dark then it indicates that your backlight is not functioning. I have searched and have had no luck finding how the backlights are powered on this tv. My thought is maybe they are powered by the power supply board. My suggestion would be to contact your local authorized service center and see about an estimate of repair.
Take a look at this link, not sure it applies to your situation. It is the fifth topic down from the top, it starts out For models...... It includes your model number.
Sorry, There is just too many variables when trying to trouble shoot over the internet. I think at this point a technician is your best bet, I don't want to steer you in the wrong direction. Good luck