No second gear or reverse 400ex

So I was riding down a path when the rear end locked up then unlocked and second gear was gone and when you would turn a corner you can hear some rattling in the cases. When I got home I put it in reverse to move it and it would not work then when I put it in first gear it was reverse then it popped back in reverse and it did that twice. I’m kinda lost tryna price out parts if it’s either shift fork or second gear