If it has gas i would check the switch/button on the clutch pedal or the neutral safety switch, i spent a couple hours in the heat just yesterday wondering why mine wouldn't turn and it was because the piece on the clutch pedal that hits the switch/button had broken so the car didn't know i had the clutch in and wouldn't start. Just an idea, it may end up being a relay or something other. When is the last time that you started it before your problem? Best of luck
EDIT: Forgot the how, for the clutch switch you can just look at the top of the pedal and make sure its getting pressed when the clutch is down. Probably a real test, and any other info you need in here
240sx-fsm-full-service-manuals-s13-s14- ... 11075.html