Stuck in 1st gear

If you had taken the shift levers off the trans, to disconnect the shifter, it's possible to push the side-cover shifter shafts into the trans (like when you hug the trans when you lift/remove it) - allowing the interlock to drop the spring/ball bearing into the trans.
I can't remember the result (missing detent), - but I'd look there, - side-cover/interlock.