is the little orange light blinking by the temp control knobIJ If so, the a_c, heater computer has set a code. there is an electric valve under the jumpseat to control hot water flow.
Most likely the electric heater valve is bad. I have changed some myself. You have to take the buddy seat completely off to get to it. When you buy the new one, make sure you get the pig tail, too. JD has changed the valve and the electical plugs don't match.