I have a Rheem natural gas water heater.
The hot water was accidentally left running in the bathroom sink overnight for about six hours.
Nine hours later the water from the faucets is cold.
Why?
Now I cannot re-light the pilot light. All of the breakers are on.
Best Answer
Given the clarification in the comments:
There are a few possible causes. The first thing to do is remove the metal plate that covers the access panel for the burner compartment (don't remove the access panel itself). You need to look for the reset switch and press it:
If you are lucky, that switch got stuck and resetting it will solve your problem. If you aren't lucky, the problem is most likely with the thermostat or the burner unit. I would recommend you call a plumber or a qualified service technician.
The burner has a thermocouple that detects the presence of the pilot light. You can test it with a voltmeter capable of millivolt resolution. There is no need to open the burner access panel to do this. Disconnect the thermocouple from the thermostat and connect the multimeter (one lead to the silver end that was in the thermostat, and the other to the copper housing). Then re-light the pilot but keep the gas button held down to prevent it from going out. After about a minute the voltmeter should read around 18mV. If it's much less, the thermocouple is bad.
The thermostat has a one-time thermal cutout fuse that may have tripped. Many (most, all?) water heaters have a special thermal limit switch contained in the thermostat that is meant to shut the whole operation down semi-permanently if the thermal limit is ever reached (I believe it is around 180F). This is separate from the limit switch on the burner access panel, and it cannot be reset. It is to protect against pressure build-up and explosion. Fixing it requires the tank be completely drained, the gas lines disconnected, and the thermostat removed and replaced.
The continuous cycling of water through the heater probably caused the burner to run almost continuously. If your tank is older and has a large amount of sediment built up at the bottom, it may be at a level where it's covering the thermostat temperature probe that extends into the tank. This might cause excess heat to be retained around the temperature probe when the burner is running for a long time, and eventually trip the one-time cutout. That is just a guess, but it seems reasonable.