From the HEET website:
Will HEET work when the gas line is already frozen?
Maybe, but not always, it depends where the gas line is frozen. You can try adding a bottle of HEET, waiting 15-20 minutes and then trying starting your engine. If that does not work, you should have your car moved to a warm garage to thaw out. To avoid this from happening, you should always use HEET as a preventative measure before temperatures are expected to fall below 32°F