You really have to sit down with an attorney knowledgeable about copyright law who can familiarize himself or herself with the facts and give you good advice.
First you must find out what is the material that they allege you have infringed. Once you recognize the material you, or with the help of a copyright or intellectual property expert attorney, should determine whether there was an infringement, even if involuntary. If none, you should answer their claim with a denial. If you have infringed I suggest you handle it through a copyright attorney and not by yourself.