Based on your description, something internal has come apart. It could be several things. For example, the forward clutch assembly, one-way clutch assembly, and so on.
Here is what I suggest: The first thing to do is remove the transmission pan to see if there are any broken parts laying in it. If there are, the transmission will need removed and taken apart to see exactly what has happened.
To remove the pan, see the two pics below. When you do this, make sure to have a large catcher under it to catch the fluid. Slightly loosen all bolts. Then select a corner of the pan that is easiest to get to and slowly remove the bolts to allow the fluid to drain from that section only. Otherwise, you will have a mess. LOL
Let me know what you find or if you have other questions.
See pics below.
Images (Click to enlarge)
Saturday, September 10th, 2022 AT 9:25 PM