Do you mean that no hot water is reaching the shower, or do you mean that no hot water is coming out of the shower. The answer to this will determine where the problem is.
If no hot water is reaching your shower, then there is probably a problem with the boiler or the pipe is blocked. You can easily test this. You can turn on other faucets in the bathroom to check whether any hot water is reaching those.
If the problem isn't with the boiler and the shower is in fact receiving hot water, then the problem will be with the shower mixer valve, specifically the thermostatic cartridge. The cartridge controls water temperature. The inner workings of the cartridge move up and down to adjust the amount of hot and cold water entering the cartridge. If it has seized up, the moving part can get stuck in a certain position. It sounds like this could be the case with you and therefore replacing the cartridge should in theory fix the problem.
Hope this helps. Shower Thermostatic Cartridge Spares