If your artwork was a name or symbol or word or something, and someone trademarked this through the U.S. Patent and Trademark office, there are tests to determine if this is a violation of trademark laws, copyrights or otherwise. If this person passed this work off as his or hers and used it to obtain a trademark, it could be voided upon the finding of either fraud or identity theft or theft of work. Further, it could be a copyright violation as well but if you did not file your work with the U.S. Copyright office, your copyright suit may only be limited to state copyright law or case law. Talk to an entertainment or intellectual property attorney about your next step. It may be as simple as letters to cease and desist to both the person who took it from you and to the U.S. Patent and Trademark office.