Hi , there is a list of probabal causes, start with the basics, see if you have a spark at the end of your plugs, see if you got fuel coming through, and check to see if you got air ie airfilter not clogged up.
if you have spark then it could be fuel starvation due to pressure build up in the tank caused by blocked fuel filler hole/vents, not allowing the fuel to flow from the tank air locks up, if after trying to start it and doesnt run, open the filler cap see if you get a hiss/woosh sound from that area, if you do then that could be your answer, try starting the bike very briefly with the filler cap open, if the bike fires up then you will have to service the filler cap vents.
carbs could need cleaning if been standing awhile as petrol go's off.
check timing
no spark :- faulty new plugs, incorrect grade plugs, coil pack defective, saftey kill switch activated or inoperative. alarm immobiliser active or defective(if fitted)
the list is exhaustive, these are just a few quick ideas to check before having to start stipping the bike right down for further investigation.
