I have an older Shimano Symetre 2000.  There is a lever that switches from instant anti-reverse to back reeling,  that lever and shaft as come completely out and just hangs from the back of the reel.  

If it works ok otherwise, sure, it’s worth repairing.  Probably just one small clip or something broke or came off the shaft.  Of course, taking the reel apart and getting it back together right may be beyond your experience, so you might want to send it to a reel tech to have it done right. 

Let me know if you need help. Should be pretty straight forward