Gas on stove left on?

Ok so one if my knobs is broken so I can't shut it off. We just turn the gas off and on to use the stove. I shut the gas off buy accidentally left one of the other burners on ignite. The whole house smells. It ws on for almost 4 hours. But why does it smell if the gas itself was on? And do wr need to worry about getting carbon monoxide poisoning?