I have just had the same problem. Assuming it is not an electric shower and the heater has stopped working it could be the same thing. In my case it is a thermostatic shower using mains water and the thermostat is faulty. As I was unsure of the make of shower I spoke to this company http://www.shower-spares.co.uk/index.html that I found on the internet and they said I should email them a photo of the shower, they would identify it and I could then order the part. It has just arrived and the plumber is fitting it tomorrow so, hopefully, that will solve the problem.
TMJ
October 2009