First try restarting your Sky box by switching off at the plug for at least 4 minutes.
If that doesn't resolve it, it's most likely the resukt of a problem with your dish setup. Unfortunately Sky no longer provide engineer support for Sky + equipment unless you have Sky Protect so you would need to engage a local satellite engineer to take a look.
Or now might be a good time to consider upgrading to Sky Q. Any problems with your dish will be fixed as part,of the Sky Q installation at no additional cost to you.
