The "Red Button" described below on the thermostat is actually a blob of paint that probably serves as a "Tell-Tale" indicator to prove to anyone if the thermostat has been reset before. Mine now has so there is no red blob there anymore, just a hole, and this could be the case with yours. All I did to reset it was use a small instrument screw driver which broke through the original red blob and operated the reset mechanism underneath it.
Paul (same one as before!), July 2009