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Recovering a domain name after a theft or loss of ownership
If it isn't in your name, you have no rights over it -in theory-. The only way of recovering a domain that you have lost, is to contact the current holder and try to come to an arrangement with them so they may return it to you. If it's too complicated or impossible, the ICANN (the organization that supervises domain names at the global level) has issued some rules regarding disputes (which applies to your case) which you can find on their website. It is long, very complicated and expensive, but this may enable you to recover your domain name if the current holder isn't of the same opinion as you are.
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