This exact problem happens often, unfortunately. It happened to me also. My former business partner hired a tech and provided him admin access to our domains. Then, I ran into a problem with my partner, who disappeared. The tech took over the business domains and is now selling them for $20k USD and refuses to give them back. I filed a complaint with the Rgistrar and provided government documents showing that I owned the company with the same name as the domains and even copies of old emails showing that domain. Their response was simply to file a complaint with ICANN's Uniform Domain-Name Dispute Resolution Policy (UDRP). Unfortunately, this can cost upwards of $2,600, depending on the tld. All that for a domain that costs some $12 a year. Good luck afterwards in trying to recuperate the fees from the thief.
The following is a good article on how business owners should protect themselves from this type of hyjacking:
http://www.virtualsilo.com/index.php/13-content-en/it-main/internet/6-control-your-website 
