How can I fix the shower?

When I switch on the shower cable for the electric shower, it displays 'ON' but no red light comes on. Hence the shower does not work. Where could the problem be? I found a little white box on wall, with a fuse in it and know that is working fine. The property has the only type of fuse box and I am unsure where to look to try and diagnose this problem.

Any advise would be much appreciated.

Thanks