I saw in your profile your have a Tremec. I did a couple of Tremec stuck in 1st gear searches. Most hits all came back a broken shifter fork inside the tranny.
All were stuck in 1st and although felt like neutral when actually not.
I'm pasting the top 3 but there are dozens and dozens with that issue.
New Tremec TKO600 won't go in neutral (stuck in a gear?) - Team Camaro Tech
TRANSMISSION STUCK IN 1ST GEAR?????? - Forums at Modded Mustangs
My tremec tko transmission is stuck in first gear. whats going on?