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Given that you changed (gas?) tanks today it sounds like you have air in the line. The water heater apparently is where the air bubble went when you turned the gas back on. You can keep cycling the heater on and eventually the air will work it's way out or you can turn the gas off to the heater, disconnect the gas line then turn the gas cock back on until you smell gas then turn it off immediately, reconnect the line, soap up the connection (to test for leaks) then start the heater back up. Of course there can be no open flames in the area. This is how we purge the air out. If you're not comfortable doing this then your only option is to keep cycling the heater on until it fires up.
Best to wait 5 or 10 minutes before firing it back up again if you continue to smell gas in the area after you put it back together
If purging the gas line doesn't work the the next step would be to check and clean the flame rod. I'd need the model number of your unit to tell you where it is.