Assuming it's an electric shower - sounds like the water pressure is too low. If the shower were to operate the water coming out would be too hot so it shuts down.
I have a Mira electric shower and basically there was no water coming out, no lights came on. To fixed it I replaced the thermal fuse and the transformer (cost about �15 in total for the parts). I don't recommend you to open up the electric shower and try to fix it. Leave it to the electrcian, unless you know what you're doing.