If it is going completely black then it will be difficult from here on out, there are two things that will most likely cause this to happen and one more that can but is unlikely....
The first two things that can cause this and are more likely to are the main board and the power supply, in order to know what board is causing the problem however you will need to be able to use a digital multi meter and possibly an oscilloscope. If you have these skills let me know and I will try to assist you in the troubleshooting of this tv. Otherwise a repairman will be required, if you take this into a shop for repair you can expect to pay around $ 250 for the power supply and $350 for the main board, you will need to decide if the set is worth this to you or not.
The other thing that can cause this but is not likely is the LCD panel itself has failed, if this is the case there is no repairing this tv because the part you would need is over $700 to replace and the set new doesn't cost much more than that.
I am sorry I don't have a quick fix or easy answer for you on this one, please let me know if there is anything else I can do for you.
If you would like any additional information or have more questions please don't hesitate to ask!
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