Dou you mean it has gone into standby or switched off completerly. If the latter try a new fuze in the plug. If it is something else please repost with more informsation.
If it is not the fuze, Sky boxes are not really a DIY job to fix. Calling Sky out will cost you £65 and they will fix or replace your box for that. You may get away for free if you threaten to cancel you subscription - but - you'll have to mean it because they may reply with 'go ahead cancel then'.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix satellite receivers.