Rheem Powervent 21VP40-1--No Hot Water

[Bada Bing]

This morning the hot water was rather cool; then later in the morning, washing dishes, the water had warmed a bit and quickly went cool; and now, after a few hours, the water is plain cold. I went downstairs to find the water heater apparently doing something (it's making a noise like it's heating and something's venting, but no hot water). No pilot light in this unit. The solid lighting unit, or whatever that's called, has a little green diode flashing on and off. Any troubleshooting guidelines? There is an on/off switch in there and also a gas on and off lever. There might be a reset button.

I'm guessing the unit dates to 1994, as 94 is the third-fourth digits of the serial number.

Thanks!