Hydrolock, that is to say the cylinder is full of fuel. Pull the glow plug and turn the engine over several times with the glow plug out until it stops spitting fuel. Replace the glow plug and try again.

There are some engines that do this. If you cannot get it to stop, with the help of a friend flip the plane over upside down and then start it. Once it is running you can turn it over and it will be fine. Be sure to think it though and make sure that you have a safe way to secure the plane with it upside down and then once its running you have a safe way to flip it back over.