Whoohoo and congrats for resurrecting your beloved machine.
Quick update on my sewing machine. Well it's made a brilliant recovery and is now fit and lives to sew another day.
It was a loose spring inside And needing a few screws tightening. Very happy now.
I had an old Kenmore this happened to. I got online and did research and a tech had said that the grease on the underside had hardened. Told me to to take a hair dryer to it to to soften it up. It worked !
It may not work for you but it's worth a try before you take it apart.
Going to have to give it a full medical tomorrow..will take it apart as best as I can
Sounds like the reverse button or lever is stuck. Maybe it has a spring inside that is jammed.