You have 2 choices, software reset or hardware reset. It's up to you what you try, but please read the advice in full before starting - if you break your box, please don't blame me!
The following is copied from http://www.ecademy.com/node.php?id=73309
This is useful if your disk usage percentage doesn't go down even after clearing programmesfilms off.
The following is offered on an ADVICE only basis, take care when making changes:
WARNING following this advice is totally at your discretion and anything which happens to your digibox as a result is your responsibility, not mine. You will lose all recorded programmes from your Sky+ Planner.
Sky tech support can talk you through performing a system restore over the phone. To do this yourself, press 'services' on your Sky plus remote, then select option 4. Next press 0, 1, select - you should now see the engineer's menu. Choose option 8, 'Full System Reset'. The box will now take a few minutes to reset itself
Try pausing or recording again, and if the problem persists disconnect the digibox from the mains power. Press and hold the 'left' and 'right' navigation buttons on the front of the digibox whilst you reconnect the mains power. Continue holding the buttons in for about 20 seconds, and when the ring of light on the front comes on, press the 'select' button on the front of the digibox (between the left and right buttons you were holding). The Sky+ recording logo lights will spin backwards and, according to the Sky engineer, this will "clear the hard drive".
Once this is done, disconnect the digibox from the mains power again and this time press and hold the 'back up' button whilst you reconnect the mains power. Continue holding it for about 15 seconds until 3 lights on the front of the box come on and you get a full-screen message saying "Updating system software. Do not disconnect mains supply or satellite dish."
Leave the digibox doing its thing for about 10 minutes and it should be fixed! Try pausing and recording things again... hopefully you've just saved yourself £65
If you've not been so lucky, you'll have to get Sky to fix it for you. Some people report that if you ask to cancel your subscription Sky sometimes offer a free callout to fix the problem - you're more valuable to them as a paying customer than the cost of an engineer callout
Sky customer service's (seemingly unpublished) freephone number. You need to dial 0800 731 6965, then option 2, option 4 March 9, Sky have closed this Freephone number
Copied from http://www.digitalspy.co.uk/forums/showthread.php?t=484098
Full System Reset via front panel:
- Remove power at mains.
- Hold LEFT+RIGHT.
- Apply power, wait for ring to light.
- Release LEFT+RIGHT, press SELECT.
- Ring cycles backwards while disk is quick formatted.
- If format sucessful PLAY lights up, if not RECORD lights up.
- Box goes into standby - wait 30 seconds are wake it up.
Warning: Full System Reset deletes all your recordings and planner entries.