Is it an auto-ignition model?
If so, there is a thermal fuse in a clear tube that goes to the circuit board that could have melted. Try temporarily removing the limiter, plug in the wire the limiter goes into and the connector on the other end of it, and try running the water heater. If it lights, you need a new limiter.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.