On most of those things, there's a small 3AG style fuse on the circuit board (it's soldered on) next to the transformer plug. There is also a large 20 or 30 amp fuse that needs to be checked If either is blown, then it'll do just what you're talking about. Could be the transformer too.
If your board doesn't look like this you might want to upload one of yours:
Several things, but that's the first place I'd look.
If it's really cold where you are and there is a danger of freeze up, I'd consider draining it until it gets fixed.