Your tractor needs a good tune-up. Make sure that the gas in your tractor isn't old. Change the fuel filter and spark plug. If you do not have the owners manual you can find the manufacturer web site online and print out a copy to assist you.
Have a compression test done, to do this you need a garge that screws into the plug opening then crank the engine, if there is no pressure, there is a broken piston rod, do this test on each plug opening.
Unless you know a lot about the complexities of a lawn mower, it is recommended that you take it to a repair shop, or just someone that knows how to work on lawn mowers. There are many reasons why it made a loud bang. You should really just take it to someone.