The CO detector goes off. Probably bad batteries, but let's get everyone outside just to be safe. Then you put brand new batteries in, and it still goes off. You have a carbon monoxide leak.
Again, you make sure everyone is out of the house, pets too (if you have the time).
Now, who do you call, and how do they normally proceed? 911 seems obvious, but in this scenario, no one was injured and so 911 might be overkill. Local fire department seems like a next-best alternative, but I live in a small town and I'm not sure if they would know what to do next.
Obviously, you need to find the source of the leak, and then call in the right contractor (furnace/heating system technician, etc.). But you can't call the right contractor until you know the cause of the leak. And if you can't even go inside your home, then how do you find the source of the leak?!?
So, who ya gonna call?