Try switching your box off at the mains, before you switch the power back on hold the back up button down on the front of the box while you switch it on, three lights on the front should light up and the stanby button. A message will appear saying updating system software, do not disconnect from the mains, etc, etc and leave it alone until it has completed (approx 10-15mins). Then give it a few seconds and switch it on using the stanby button.
This worked for me, good luck.
Sam, January 2008