Check out the size on the cable wired to upstairs, it should be on the insulation sheath if you can find a section. Also check the kw of the shower that is fitted if recent years I would suggest likely to be 10.5kw shower and depending on cable run and method will depend on size required. It is certainly not a 10mm which is the best to use , I am even doubtful it is a 6mm. The MCB have a bio metal strip inside which if overheats / over loads will trip the mcb (doing its job), if you have a 10.5kw that is def your trip problem and cable may need upgrading, if the shower does not even turn on that is another issue, could be a cable has come loose or even burned out somewhere. Could be many other things bit worth getting resolved to use the facility.