Ask the person who organized the entity. If the organizer is an attorney, he/she may know. An attorney who organized the entity may have drafted your Operating Agreement, as well. If it was one of the members and that member is nowhere to be found, then there is not much you can do. Contact the IRS as they may have information regarding your LLC tax filings that may lead you to a better answer or enlighten the situation.
This begs the question of how you are a member of an LLC without knowing anyone else involved, any of whom might have copies of the documents. Whoever filed the Articles is your first and best bet, as that person (likely an attorney) might/should have the operating agreement. What does the LLC do? Does it have a place of business you can go to?
To questioners from West Virginia & New York: Although I am licensed to practice in your state, I practice on a day-to-day basis in Massachusetts. I answer questions in your state in areas of the law in which I practice, and in which I feel comfortable trying to offer you assistance based on my knowledge of specific statutes in your state and/or general principles applicable in all states. It is always best, however, to work with attorneys and court personnel in your own area to deal with specific problems and factual situations.
If you had several members, your accountant or tax professional should have issued K-1s to each of them at some point.
The foregoing is not legal advice nor is it in any manner whatsoever meant to create or impute an attorney/client relationship.