My husband fitted a new thermostat in hot water tank approximately 3 months ago. However, yesterday 4th Jan. water stopped heating up.

Your thermostat should have a reset button on it ,try pushing that

Hi There is small RESET button on top of thermostat-try that if that will not help immersion heater may need to be changed-or thermostat.

It maybe just that the element has gone? If the thermostat has a reset but as has already been suggested try pushing that, although some cheaper thermostats don't have this safety feature! It could also possibly be a loose connection where the wire enters at either side of the thermostat, so worth checking that they are still tight.