Hello. On Saturday after I filled up for $2.49 a gallon

I hopped back in and couldnt get out of first gear. I crawled through the mountains of Tennessee for about 20 miles before calling AAA and having it towed home.
I've heard of trannys getting stuck in gear from fluid being spilled through the shifter and frying the control module but nothing has been spilled to my knowledge.
I just had it serviced so Im assuming im full of tranny fluid, but without a dipstick, I cant tell for sure.
Any thoughts?
Thank you -
Roy Andrade