Firstly what make is your box? if its a thomson it is likely that the PSU has failed. However one fix I do know is to do a HDD reset and then a forced software download.
Start by turning the power off at the wall, then hold the left and right buttons (either side of select) on the front of the box and hold firmly for around a minute. The circle at the front of the box will illuminate, release the two buttons and press the select button. The light will now rotate anti clockwise, once this has finished turn the power off again and hold the backup button, turn the power back on and wait for all lights on the fron t of the box to illuminate. "Updating System Software" will be displayed on the screen.
Now if none of the above actually happens then the PSU is faulty, unless you are good at soldering i suggest going to www.satcure.com where they do replacement PSU's on an exchange basis.
hope this helps!
Andy Brown, May 2010