Hi, 
Well can explain the problem.  Either: 
1) The thermostat has short circuited, meaning the thermostat is reading too hot wehn it's not.
2) The thermostat has moved and is resting on the hot water pipe, again causing a false reading of too hot.  
Either way, I think you will need either a Redring agent, or a new shower.
                
 Lest
                
 October 2008