Power board inside the LCD or Plasma Tv my friend.
Based on the brand, whatever it is in about three to four years you can expect a tv to do this.
It`s the most common part to fail, because of where the power board is placed in the tv it`s self.
The good new is for most, all you have to do is open the back of the tv up and locate the power board number found on the board it`s self and the tv model number along with the size of the lcd screen it has.
A quick search on e-bay and you can get most from about £20 upwards.
Of course it`s down to you if you think it`s worth fixing.
The cheaper option is to open the tv, look at the power board and look for any faulty capacitors on the power board.
Typically you know when a capacitor is...