Hello and Welcome to our forums.
The power board you have uses SamXon brand capacitors which are well known to fail with no visible signs of failure. The type problem you are having indicates that the backlight circuit is shutting down but the logic board is still functioning because you still have a picture just no light. The causes of this are bad capacitors, bad solder joints on the transformers or backlight plugs, shorted driver chips that run the back lights, bad transformers or the back light tubes themselves. The first thing to do is resolder all of the connections on the back light sockets and the transformers that are next to them ( no cost to try first). Try the unit, it you still have the same issue then replace the capacitors. If that does not solve the problem post back with updates and we will go from there.