Pixellated pictures on live TV are often the sign of a signal fault. If you've rebooted your box once, then there's likely a fault somewhere, so call VM and report it. You're likely to need either an engineer visit or a replacement box, depending on what VM's remote diagnostics turn up. They can do these while you're on the phone to them.
You've mentioned this only affects one of your TV boxes, I'd recommend swapping the boxes around and see - does the fault follow the box (and prove a fault with it), or remain with the same room (and suggest an issue with your internal cabling)
Once you know which one applies, the quickest way to report your fault is to call 150 from a VM phone, 0345 4541111 from any other phone - choose the options for "I have a fault with my TV service"
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Thanks for taking the time to raise the issue with the picture quality on your set top box with us here at the forum, we're eager to get this looked into further for you.
From looking at the equipment and levels, there does appear to be an issue which will require a technician, I have provisionally arranged this for Saturday 17th April between 8am and 12pm. Please let me know if there are any individuals shielding at the property or if this isn't convenient for you and I can look to change it to the next available appointment.
Thanks for organising this. I've just learned that there will only be someone at home until 10.30 tomorrow. Is there are chance of making sure the engineer can arrive in time, or perhaps rearrange for next week?