Before making any pre-diagnostics we need to know what hot water system supplying your shower ,example is it a combination boiler or a conventional gravity hot water cylinder ?
Let's say the latter! Then it could be 1) an air lock .2) low pressure
Also it could a pressure booster pump failure if you'have one .
These are few causes to begin with .
I hope these few answers can help you at least determine the fault .
You can get back to us if further help is needed .