Thank you very much Autumn Snowbird!
When I open the relief valve on the outside, no water comes out. The water pump is full and it has stopped running, and when I turn on the hot water taps water comes out but it's just not hot water... Ice cold. The water heater has water in it, because I'm getting water out of my hot water taps. Also, I pulled the drain plug on the water heater, and water pours out of it.
I'll try the other suggestions that you have, but right now it's not igniting and the light's not even turning on. If it were a nest, it would seem like it wouldn't turn on at all, but it turned on last night... and this morning... it then shut off. Update: the burner tube is clean.
