This is a common issue with JK's. I call it the "random no start club" and I'm not only the president, I'm also a member
I bet you have an automatic...the issue is cause by the sensor in the transmission that tells the computer you're in park. If you turn the key and it just clicks, I move my shifter to No Gear and it starts flawlessly.
You can also jiggle the shifter while in Park to engage the sensor...or get a new sensor.
I'm out of warranty, so I live with it. It only does it once or twice a month. I've never seen a manual with this issue yet, but if one did show up, I bet it's the sensor that relays the message to the PCM that the clutch is engaged.