Also, Here is what .bomb says:
What should I do if I've lost my ticket?If you had a paper ticket, please call Reservations at 1-800-UNITED-1 (1-800-864-8331) or outside the United States visit United worldwide contacts. Also visit International ticket offices. You will need to fill out a lost ticket application at any United airport ticket office or through your travel agency, and charges will apply.
If you had an e-ticket from United, you are a Mileage Plus member with a united.com profile, and you included your Mileage Plus number in your reservation, please log in to My Itineraries. Please note that if your itinerary does not appear, you should contact Reservations regarding your lost ticket.
If you purchased your ticket from another website, please contact your original booking source.