The Aqua Esperit mod no 53-592430 shower is not working?

The shower is not working.
Serial no 04 120612
The shower is not working:- The electric supply to the shower is ok. Tested the
Current/ supply to the, what looks like a water valve (where the water supply comes into the shower) this is a white plastic part and that shows supply through this part is negative so not working. Any ideas where I could get the part and, if possible the proper name and part number would really help.
Thanks in advance.