Thanks for the quick reply to all of you - I'll start poking around.
We did do some work on the basement a while back (that's where the
wires run through), but I can't think if things started happening
around then or not (I think I would have noticed), however I could
imagine some of the work that was done may have messed with the
wires. I do think that my thermostat is on the same set of wires and
it appears to be working fine so I'll start working from the button to
the chime. Thanks!