This is a tough question to answer because there might be different answers depending on the way your property is set up.
Heating controls are usually independent of an electrical shower so changing the shower should not alter the settings to the heating, but if the better back up in the time control isn't working it might have returned to the factory setting when the power was switched off.
The isolating switch is not supposed to switch itself off you should have to do this manually. It might be that the fault was not with the shower initially if you have lost power to the new shower.
As for the shower it's doubtful that the shower needed to be replaced there are at least five parts that can be replaced that normally fail after nine years.