First question is whether the tank gets hot but the water at the faucet does not, or neither gets hot. Presumably if the heater "fires up", the tank gets hot, but need to verify that. If the Pressure Relief valve on the tank is hot to the touch, you can assume the water inside is hot. Also, is the water at the faucet room temperature or warm but not hot. In other words,is there partial hot water flow.
If the tank is hot, then you have a heated water delivery problem. The likely causes have already been mentioned, bypass valves in wrong position or a stuck check valve at the tank exit or entrance. Either prevents hot water from reaching the faucets. Sometimes you get a brief flow of hot water but it quickly gets cooler. That indicates cross flow, where water from a cold line crosses over into the hot. This can happen in any faucet that mixes hot & cold and is left open, e.g. the outside shower faucet.