Not a lot to go on.
What make is your water heater?
Is the water heater full? (can you open the relief valve and water comes out)
If it does, is the water hot?
What do your bypass valves look like (if installed)? 3 valves? 2? 1? none?
Is the AC circuit breaker closed?
High limit reset buttons in?
Does the gas fire?
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW