Most likely a fault with your dish setup, either misalignment, a faulty LNB, or a problem with the cables.
You could try disconnecting and reconnecting the dish cables at the back of your Sky box (make sure the box is turned off at the plug). If it doesn't help you'll need a book a local satellite installer to investigate, unless you have Sky Protect.
Alternatively now might be the time to upgrade to Sky Q - any dish issues will be put right at no additional cost as part of the Sky Q installation.
