Looking for a little help TH400

[AMSOILGUY]

Yesterday I was driving my car just running an errand, or I should say I was planning on doing that. I only made it about a mile from my house and the tranny locked up. What I have is a TH400 with reverse manual valve body. So this is how it happened. I was coming up to a red light in 2nd gear and slowing to a stop and shifted into 1st and it locked up stopping the car. So while the light was red I put it into neutral and tried to put it back into 1st. Normally you can feel the car go into gear and it didn’t do that. So I tried 2nd and thankfully it would at least role forward in that gear. After determining that I at least had 2nd I put the car in park. I tried again to put it into gear and it just acted like it was in park. So I drove home in 2nd got into my driveway put it back in neutral and tried 1st again with the same result. Then I was like well lets see if I have reverse. I did and then after backing up I tried 1st gear again and I’ll be damn but it engaged and I pulled it into the garage. While I had it running I checked the fluid level and it was spot on.

My first thought is that it has to be a linkage problem but I know very little about auto trans so what would cause this? Of course that little trip to the grocery store was all the attention I had time to give my car yesterday so it has sat since this happened but I will have time today. Any help is appreciated.
Thanks
Jacob