Assuming you mean live TV channels and that you've already restarted your boxes once, that would suggest you probably have a fault with your incoming TV signal - which needs reporting to VM in order for them to (most likely) task an engineer visit.
If both of your boxes are affected, it would suggest you've either been unlucky to have 2 boxes fail - or there's an issue on the common cabling somewhere.
I would recommend you call VM to report your fault, 150 or 0345 4541111.
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