Somebody infringed my copyright. What can I do?

You can complain, get an attorney to send a cease-and-desist letter, or, ultimately, sue.

“A party may seek to protect his or her copyrights against unauthorized use by filing a civil lawsuit in federal district court. If you believe that your copyright has been infringed, consult an attorney. In cases of willful infringement for profit, the U.S. Attorney may initiate a criminal investigation.”
— U.S. Copyright Office