If your shower water goes from hot to cold constantly and it's not that you've run out of hot water in your water heater then the next cause of a shower going from hot to cold and back again is usually due to the thermostatic mixing valve.
The mixing valve that mixes hot and cold water in your shower could need adjustment or could be going bad if you have a thermostatic mixing valve in your shower.
Other than that you could be running out of hot water in your water heater if the hot water doesn't immediately come back.