Originally Posted by InspectorDread
You probably have air in the gas line. Loosen the gas line connection to your water heater and let it bleed out until you smell the propane. Retighten the connection and hit the igniter.
There is a safe way to do this. The above statement will work, but once you have flared the copper and seated it to the brass loosening and re-tighing it is not good for the seal. Stuff can get in and cause a leak
If the heater is a pilot light model use a butane igniter and do the normal pilot light functions, BUT keep the pilot light knob on for 30 seconds, ans keep the igniter lit for 45 seconds. Turn the knob over to on, with the igniter lit. This will open the main valve and the main burner and pilot will light.
If it is a DSI model you will need to do 4-6 restarts.
In either case be sure you have gas to the cook top first.