You can choose an alternative Domain Name, or if you feel that you have legal rights to a particular Domain Name you could follow the Uniform Dispute Resolution Policy (UDRP) and try to regain you Domain Name.
For example, you believe that you have legal rights to the domain name 'tonismith.com' but it is has already been registered by someone. The Uniform Dispute Resolution Policy will inform you how to pursue your claim. To view thew UDRP click here.