Can't shift into gears! HELP!

I have a 91 Jeep Cherokee 5spd manual inline 6. I was driving my Jeep today and I came to a stop sign then waited for traffic to clear and tried to shift into first again but nothing (no engagement) What I ended up doing was leaving my jeep on the side of the road and tested it a little. With the engine on, the gear shifter is stuck in neutral and wont shift to any gear. Well you can shift it into reverse and it grinds. With the engine off you can move the shifter to what ever gear you want. Please help thanks!!

Forgot to mention that when I tried to engage first gear there was a rather loud bang from the tranny.

Sounds like you have lost your clutch. Do you notice any difference in clutch pedal tension or is the pedal easy to move or harder. If you look at the clutch linkage either you are missing a rod or something has come undone. Had this happen on a Chevy but not on my jeep (mines an automatic) but the Chevy had lost a clevis pin and the clevis had slipped down on the throw out lever. Try those things and tell us what you found.