It really depends on how it is working now and what caused the fluctuation in temp. If the shower refuses to heat up, it may be the over heat stat has triggered. This is non resettable and needs replacing.
It is difficult to offer advice without information about what happened to cause the fault.
Repairs to electrical showers are not for the novice so unless you have at least some electrical experience, it would be wise to seek help from a qualified person.
Thank you for your advice. However, it's now working okay. What I didn't say was that my area had a burst water main and I think it was something to do with an airlock. I unscrewed the shower head and there was quite a bit of water in the hose. I cleaned the shower head and it seems to be working fine now! And the 'Low Flow' light is no longer coming on.
I am glad the shower is now working, but it is really frustrating you posted the question without bothering to include important information.
If you had said about the burst water main, I would have explained that because electric showers control water temperature by altering the flow, a sudden drop in pressure caused by the burst would have caused the water temperature from the shower to rise dramatically, and then the low flow would cause the elements to be switched off causing the freezing temperature you described. Simple really.
We have the same shower and the same problem- have worked out that the problem only appears when water pressure in the area drops. Sometimes lowering the shower head's position relative to the unit seems to help.