Is your loan listed in the NSLDS?
Its absence doesn't prove that the loan is exempt from the statute of limitations, but its presence would suggest that it is exempt. It's unfortunate that you don't have a copy of the note.
Given the unusual background facts you present, I am left wondering if the loan documentation still exists and is available. You do have the right to instruct a collection agency not to contact you, although they can still sue you.

