Bottom line is ...YES, a bad plug will cause it not to start... HOWEVER; if you checked and your spark plug IS getting spark...then NO, the spark plug is not your problem..
..take out your spark plug and pour a tiny amount of gas in the cylinder, put the spark plug back in and try to start it..if it fires and then dies, you have either a carberator issue or a fuel line issue..
...if all else is good...time to check the head gasket, intake gasket, piston rings, etc....