Okay...it's that likely to be true even though the fire initially lit (and then fired some time later...although I still heard the fireplace humming)? It no longer lights though.
There may be a problem with the gas valve itself. If there is a pilot flame and the unit will not light there may be a problem with the valve or an issue with the pilot sensor that goes to the valve.
Okay...still no worries about carbon monoxide though?