Am not mchops, or a plumber, but a couple of questions:
- Electric shower or running off a boiler/hot water tank?
- Does the temperature dial feel different when you move it, either spinning more freely or harder to turn?
If it's electric, probably a new shower unless you know a very good electrician who can repair it. If it's running off a hot water source, then hopefully just needs a new mixer. Whether the tiles need to come off depends on what type of controls you have, mixers can sit in front or behind them.