If your Atwood water heater is a combination propane/electric model like mine, the control panel has two switches with a light in between. One switch turns on the electric heating element, the other turns on the propane heating element. The red light in the middle is a fault indicator. If the light comes on, that means the water heater experienced some kind of fault and turned itself off.
First thing to check, is the water heater full of water? Easy to check, turn your galley faucet all the way to hot, then open it. If water comes out in a steady stream, no spits or spurts, the heater is full of water.
Next, check the water heater fuse. Both the electric and propane heating use 12v
electricity; the propane side uses an electric spark to light the propane flame. If the fuse is blown, neither side works.
Next, see if the water heater is having the same problem on both propane and electric. Try it on one, then on the other.
If it acts the same both ways and the fuse is good, the most likely culprit is the thermostat. That's what was wrong with mine, a defective thermostat, caused by a wire being crimped when the insulation was installed.
On the other hand, if it works on electric but doesn't work on propane, or vice versa, then something else is wrong and we'll be getting beyond the limit of my personal experience.