sadly this is quite common. The most usual cause is that one or more of the wires in the switch was not tightened sufficiently. Copper wire is a rather soft metal, and these large sizes are stranded. This means that after the terminal has been done up, the soft metal deforms away from the clamping pressure and the connection becomes loose. If you do these big connectors up very tight, and then the next day try again, they will often tighten down another half-turn or more, and maybe again if you try them after a week.
Once the connection is loose, it has spots of high resistance, which overheat. The heat makes the metal even softer, so it carries on and gets worse. It also scorches and burns away at the spots of high resistance, until it no longer turns on. Electric showers use a very high current.
What this boils down to is that the ends of the cable are likely to have been damaged, and so are the brass terminals in the switch.
Sadly it is not possible to just tighten the connectors now, as they have been damaged by heat. You can buy a new switch easily enough, but the damaged ends of the cables have to be cut away (they will probably have gone black) to bright undamaged metal. Unless the installer left plenty of spare length on the cables, it will be necessary to crimp on an extra piece. This requires a special tool (and the skill to use it correctly) so it will be more economical to engage a local electrician who is a member of a self-certification scheme, and who personal recommendations tell you is reliable.
A good-quality switch by MEM, Crabtree or MK will be better than a cheap brand, but the most important thing is clean, bright, undamaged copper, and terminals tightened and retightened.