How can I fix my LCD TV?

My TV is not firing up when I turn it on, the red standby light comes on and the green 'on' light displays but it doesn't show any picture or sound. It has done this before and after a day it seemed to sort itself out but is there anything I can do to fix it now? I've tried turning it off at the mains again but it's just stopped working. It is a Phillips flat screen LCD 42"TV and we bought it as a re-conditioned model. When it was repaired before they said it was some sort of short circuiting in the computer board but now it's guarantee has run out and if it doesn't decide to start working by itself I don't know how to fix it. Hoping someone can help.