No hot water

i have one valve for bypass that stops all water from entering the heater. looking at the back of the heater i could see one other valve(yellow in color if that makes a difference). i believe that the mixing valve was replaced once under warranty. i do not know if they could access from underneath or took out the heater(which is probably easier than taking out the toilet). there is also a chrome (i will call it a T valve on the balk of the heater) valve and of course an inlet and an outlet. i could not reach the yellow valve(again without taking out the toilet). i was hoping that opening and closing(or vice versa) would loosen it. i did winterize, but the only valve was the bypass that i turned--the tank was then drained and plug returned with only one turn(which would not seal the opening) i have an outside fauce and it is closed.