Alasdair Taylor's Answer

Whether you have any rights to the domain name and what the best way to get control of the domain name might be will depend upon information you have not provided.  Important factors affecting your rights will or may include:

• the domain name itself;
• any written agreements between you and your business partner concerning the domain name or any trade mark reflected in the domain name;
• the ownership of relevant trade mark rights, whether registered or unregistered;
• the history of the domain name, including the uses to which it has been put;
• your past and present relationship with your business partner;

… and so on.

You should really speak to a specialist domain name / internet / IP lawyer about this.  In order to get sensible advice you would likely need to disclose confidential – to some extent – information.