Immersion thermostat reset button?

Hi

My hot water is supplied by immersion heater only (no central heating). I have this on a timer set for 2 hrs morning and night.

I recently fitted a new timer as the old one had packed up, and the heater has since cut out and stopped heating water. I have found that this can be overcome by pushing the reset button on the thermostat, but something else must be wrong.

Any ideas anyone? Thanks in advance for any help