If you've restarted your Sky box, by switching off at the plug for 5 minutes and the problem persists, it's most likely because of a problem with your dish setup.
Unfortunately Sky no longerprovide engineer support for Sky+ boxes* so you would need to get a local satellite installer to come and take a look. Or upgrade to Sky Q and any dish problems will be sorted out at no additional cost as part of the Sky Q installation.
*edited to add: unless you have Sky Protect.
