You need to check for spark and fuel as a first point of call.
Spark. Take plug out and pop in HT cap. Put end of plug on metal frame and start as normal. this will earth it and should spark! Just replace plug then you know it is ok but still check for spark.
Fuel. Pop a bit of fuel down bore hole when plug is out. pop plug back in and start as normal. Does i fire for a few secs?
bb
December 2010