They should do it for free, especially because you can smell gas in your house.

They will do a very thorough inspection and give you a sense of what could have caused it. Armed with that info, you can probably go back to the guy and explain what happened and demand they fix it.

Gas companies usually have such folks on call 24/7, so call now.

If the original contractor continues to refuse to do anything, get someone else out there asap--you need your heat back on, otherwise the house could start to get damaged--and pass the bill along to the original contractor and suggest they tender it to their insurance.