If you disconnect every wire from the trans - it should default to O/D, meaning it would be stuck in 4th gear if you selected D, no shifts at all. If you have zero drive in D or any position of the selector then check that the oil pump is actually working, remove a line from the radiator cooler and see if oil is actually coming out.
If the pump is working and you are missing drive, filter is clean, fluid is good and GENUINE - not mitti ka tail mix with engine oil, then seems that the 1st brake piston is bad in the trans - overhaul required.
btw yard transmissions are not fresh, they can range from OK to garbage.
p.s. Can you hear any clanging sounds? Do a check when you remove the torque convertor, get a spare input shaft from the wrecker or someplace, and stick it in the convertor nose and verify that the splines are not broken off - seen that happen in a mitsubishi galant transmission.