Have you had high wind which could have moved the dish out of alignment? Or could it have been knocked for some reason?
You say you've checked all the cables, I'd suggest disconnecting at each end, scraping the connections clean, and reconnecting making sure it's all tight.
LNBs can fail, although it's rare, so your next step assuming you've done the foregoing, is a new LNB (they're not expensive).
Good luck.