[Dick D]

I recently had this same problem. I took a shower and everything was fine, A few minutes later the DW went to take a shower and suddenly no hot water.
What I found is that there is a check valve on the hot side of the heater (red line) where the line hooks to the water heater. what you need to do is unhook the red line at the back of the heater, then turn on the water for just a second, this will cause the check valve to pop out far enough for you to get hold of it (it looks just like an a car engine valve but only about a inch long and made of white plastic) with the line off, you may be able to feel the top of the valve on the heater side of the connection.
Oh, and you will get some water it the basement so you may want to cover your converter so it won't get wet.
It took me several hours to find this, but only 10 minutes to get back in operation after I figured out what the problem was. It's a bad backflow preventer. After I got the valve out, I actually had better water pressure than I had before, so I am going to leave it out.