Best way to test if it's the TV or Sky box is to tune it to an analogue channel (doesn't matter if you use the analogue or not) or play a video tape to see if it's green on that. I've heard of some Sky+ boxes developing a solid green screen (can only see green, no picture at all) and some have vertical lines. Usually Amstrad boxes, but occasionally Pace. If it turns out to be the box, call Sky and arrange for a replacement to be installed. I'm on my 3rd Sky+ box in 18 months, could be a 4th before the year's out as this one keeps crashing (same as the 2 before it).
You might like to try reinstalling the STB firmware. It's well documented in the handbook that comes with their receivers. Something like - unplug for 10 minutes, reinert plug whilst holding the backup button on the STB, watch screen for text saying leave for 10 minutes. But check before you try!
I once had a Green Screen with BBCi but it had text reporting an error. Likewise I've had a red screen.
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