This is exactly the problem I had. Having looked at various answers on here for "won't come out of standby" I did this and, up 'til now, it has been okay:-
Switch off at mains.
Leave for a minute or two.
Press 'Backup' button on the sky box and at the same time switch on at mains, keeping the backup button pressed in until the three lights (remote, message and another) come on.
Release the backup button. A message should appear, saying "updating software - do not switch off - this could take up to 10 minutes".
After this, the box should appear to go into stanby. I then switched off at mains for a couple of minutes, then switched back on - and it worked.
I hope it works for you too!
Richie Rich
February 2009