I think Petrus has it here... Seriously, try push starting it.Did you try a running/push start?
Key on, switch on, fuel on (I forget every time I work on the thing), passenger pegs up (trust me), choke on...
Stand on the left (clutch lever) side, put it in 3rd gear, pull in the clutch, push it up to jogging speed, step on the foot peg (don't hit the shift lever), dump the clutch... just be prepared to hit the clutch again (and front brake) so you don't lug the engine or stall it from being in third. I don't even throw a leg over when doing this any more, I just step on the peg and stay on the clutch side of the bike.
If it starts, you probably have a weak battery. If it doesn't, then look at some of the other more serious stuff.