Sounds like it could be the engine or the transmission. My guess by the sound of it is that the transmission took a @#$%& on you...
Pull the dipstick out of the tranny and see where your oil level is and what the condition of the fluid is. Dark/brown color and a burnt smell will probably confirm that the transmission is toast. Pinkish/red fluid is usually OK.
If the transmission oil appears fine, then try turning the engine over by hand with the car parked and the transmission in neutral. You should be able to tell if the engine has seized. Also, I would pull the starter off and check to make sure the ring gear is still in tact and that nothing is stuck in the starter gear.
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1985 300ZX n/a
See my Z project