Unplug it. Not just switch off unplug it. Then wait 30 seconds or so and plug it back in. Wait for it to boot up again(you will get a message). There has probably been a sky update or upgrade and just like a pc it needs to be switched off and booted up again.
Well it could be anything (for example for me once it was when a neighbour had cut through our Sky cable!). The first thing I always do when Sky plays up is to turn it off at the mains for 10 seconds or so and turn it back on again.