This sounds very much like what has happened to our lawn mower, minus the smoke from the carbuerator. Our mower is new, it has only been used 2-3 times when this happened.
I did find a very narrow belt (not broken) in our yard. Is this a coincidence, or could we have lost a belt that would cause this?
I have been afraid to use it since it did this last weekend. I hope that you have some ideas :)