Why is there no hot water to my Wickes Aquatronic Ultra 3 electric shower?

I have a Wickes Aquatronic Ultra 3 electric shower which is less than 6 months old. We've had no previous problems but suddenly water is coming through cold. The trip switch for the shower is on (I've tried switching off and on again). The light on the switch outside the bathroom is on, and the light on the shower unit itself is also on. Any ideas? Thanks!