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.

Below is a photo of the film that the carburetor should have if the gas has broken down.