So I have a 89 Cummins all stock, automatic. Yesterday I went to accelerate to get out in-front of a car and there was a pop and I could not go, the motor still runs fine there was no grinding of the trans or anything just a pop almost like it slipped out of gear but no gears work it’s basically like every gear is neutral. Before this happened I had slight trouble getting into second gear and sometimes hard shifts.Does anyone have any ideas as to what this could be

 

Broken input shaft?
If the flex plate broke, I would assume you would hear lots of angry sounding noises in gear.
If the driveshaft [or shafts if 4x4] broke or fell off, I would expect lots of noise as well.

Pull the inspection cover and see if you can see anything...Otherwise I am thinking pulling the transmission is your next option.

 

Or maybe filter plugged or oil pump broken.

Had a GM auto that did exactly what the OP describes- lose valve body bolts.

I think that I would check the fluid level, if OK remove the oil pan and look for debris.
If 4x4 it could also be the transfer case- easiest check- put truck into park, no parking brake and try to move it- more than 5 feet- no power transfer between axle and transmission.