Identifying a possible water heating unit issue? You will want to go throughout your house, testing the hot water components to see whether your cool shower was a remote event or is common throughout the entire house. If you are not getting any hot water, your water heating unit is the most likely root cause.
First, check the temperature setting on your hot water heating unit. If all is well, the next thing to think about is whether a fuse has actually been blown or tripped if you count on electricity to warm your water. A fuse can be easily replaced. If you have gas, check to see if the pilot light went out. If it is, you can easily relight it, following the directions in the producers handbook.
If dealing with the temperature setup, fuse or pilot light does not repair your issue, chances are you might have a range of other issues such as a faulty thermocouple, flue obstructions, sediment buildup on the heating element, or even a faulty temperature control.
When it is an isolated shower issue? If the other components in your house produce hot water and stays hot for a length of time, your shower is a remote issue. There are a lot of problems why your shower loses hot water