I found this at the Tunze site:
Failure: The pump has seized, and the driver of the Turbelle electronic sounds a continous tone.
Cause: The drive unit or the impeller has jammed due to wear, lime or snail housing, for example.
Remedy: Remove the foreign body. Clean the pump, and replace the drive unit, if and when necessary. The pump will start operating again automatically after a delay of 20 seconds.
I'll look at it in the morning. I don't think its due to wear since I've only had the unit 3 months.