Sounds like it might be the neutral safety switch.
If it does it again, try 4 low right away. Pretty sure most TJ's neutral switch is set to by pass in 4 low.
There should be write ups all over the place on how to bypass it. If it does it again, look into that.
Otherwise, have someone listen at the starter. If the solenoid is clicking when you hit the key, the starter may be starting to wear out.
If you tap it with a hammer and it fires up, thats your problem.
To replace the starter, get an autovalue one and keep the receipt. They go often, and autovalue has good warrenties.
HTH