lawnmower backfires wont start ?

i have mcculloch m56 875 dwa self starter briggs and stratton engine i have just changed the blade which involved tipping machine on its side when i tried to start it ran fior couple of seconds then stopped with enormous backfire, when tried again it just backfired, there is oil and petrol in machine. waht can i dfo. have i flooded engine or is something else more sinister involved