If your heater isn't firing there are a handful of things the homeowner can check before calling a serviceman. 

1. Check to make sure the gas valve is open. The valve is open when it is inline with the pipe.  

2. Ensure the breaker is on and that power is going to the heater. 

3. If its connected to automation, make sure its enabled

4. Make sure there is adequate water flow. If the baskets or filter are too dirty, the heater wont work.