Your going to want to post more info than that if you want people to respond...
Has it been sitting for a long time, does it have gas, is the battery good if so how did you verify, is the kill switch on, is it in gear, was it dropped, was any electrical work done recently or ever.
Just a few things that will help people give you better ideas on where to start
But if u want things it could be with no info
bank angle sensor
On or faulty kill switch
No gas (you laugh but I've seen it)
Post up some more detailed info and alot more people will.jump in and help you out
If it was the BAS, Kill switch, or Kickstand sensor it wouldn't turn over at all, just make a click when you hit the ignition... I'd say either, shitty battery, bad regulator (although it usually will start with a bad regulator, it just wont turn back on immediately once you cut the engine off) stator, or spark plugs not firing!