Hi Helen – this happened yesterday morning but appears to have happened again today. Today I haven't had time to look into it, but yesterday the fix (or coincidentally it starting to work when I did this!) was:
1. Turn off at the plug for a while (a minute or so)
2. Turn it back on
3. Wait at least 4 minutes
4. Press the 'Sky' button
5. It will come back on and do it's business
Just turning off and on wasn't enough and I got the above from a googlisation.
I know some others have had the same problem. Let me know if it works and I'll try it on my system when I'm back home.