If the smoke is white then the motor has too much oil, and its transferred to the muffler. This shouldn't affect it too much, and just need to be left running for a bit to burn all of the oil in the muffler.

If its black then the oil needs to be replaced. The usual culprit for the motor not starting is the carburetor.

If left for roughly a week with gas still inside, the gas will begin to breakdown because of the ethanol inside it. This will clog up the carburetor and cause the engine to stop.

Try to remove the cup of the bottom of the carburetor and check to see if the gas inside is clean or cloudy. If it cloudy, it will likely have also left a layer of varnish-like film over the entire cup and that will be clogging your engine.

You can remove the carburetor and clean it, remove the old gas from your tank as well, and refill it with clean gas. If you don't feel comfortable taking it all apart, take it to a service center to have it checked out.