The cause is most likely a fault with your dish or cables. Unfortunately Sky no longer provide engineer support for Sky+ equipment unless you have Sky Protect, so you'll need to contact a local satellite engineer to take a look.
Alternatively now might be a good time to consider upgrading to Sky Q. Any dish or cabling issues will be addressed as part of the Sky Q installation at no additional cost to you.
The problem may (but less likely) be a fault with your Sky box, for which the only solution would be to upgrade.
