The loss of rings is an automated process. If a ring requires the manager's attention for an exceptionally long period of time and the manager doesn't tend to the ring, the system will seize it and make it "adoptable."
To regain the ring, you should watch for it to go back up for auction. Click the Auctions link found at the bottom of every page to access Auction Central.
If the ring already has another manager, your chance to regain the ring has expired.