Hi,
If you have the engine off, does it feel like it is shifting as it always did? It could be the pressure plate, but usually, that will make some noise. Are you hearing anything?
The transmission you indicated is an all-aluminum unit. There really haven't been many issues with them. So, the best thing we can do is pull it and inspect the pressure plate, clutch, input shaft, and so on.
If you look at pic 1, I found a technical service bulletin related to not being able to be engaged. Pic 1 came from the manual under technical service bulletins and indicates possible causes of the problem you're having.
The remaining three pics explain how to remove the transmission and clutch assembly. I hope this helps. Let me know what you find or if you have other questions.
Take care,
Joe
See pics below.
Images (Click to make bigger)
Friday, May 21st, 2021 AT 7:07 PM