Most likely the power supply board needs to be replaced. It could also be the backlighting. A few TV's have a separate power board just for backlighting, but not many.
If you don't know what you're doing, DON'T OPEN THE TV UP TO FIX IT, THERE ARE HAZARDOUS VOLTAGES ON THE POWER SUPPLY BOARD, EVEN WEEKS AFTER THE TV IS UNPLUGGED!
If you don't know how to do it yourself, find some local Electronic repair shops, get quotes, try to find reviews on the shops.
Answers & Comments
Most likely the power supply board needs to be replaced. It could also be the backlighting. A few TV's have a separate power board just for backlighting, but not many.
If you don't know what you're doing, DON'T OPEN THE TV UP TO FIX IT, THERE ARE HAZARDOUS VOLTAGES ON THE POWER SUPPLY BOARD, EVEN WEEKS AFTER THE TV IS UNPLUGGED!
If you don't know how to do it yourself, find some local Electronic repair shops, get quotes, try to find reviews on the shops.