Reports of this are not too uncommon - it's an easy way for unscrupulous site operators to use your content to gain traffic from search engines.
Hopefully you included a copyright (©) statement in all your files. In the U.S., this is not a requirement, however, in many other countries it's required in order to attach copyright protection to your works. You can also take the additional step of registering your works with your country's copyright office.
Though commonly referred to as theft, legal precedence does not equate copyright infringement as theft, conversion, or fraud. However, both civil and criminal actions can be pursued. If your website included trademarks, this would also encompass trademark infringement. The same is true of other forms of intellectual property too.
In the U.S., you can report claims of intellectual property "theft" to various governmental agencies, who may have access to resources that the public does not, as listed here: Where To Report Intellectual Property Theft If you're located outside the U.S., do a search for the options available in your country.
If this is resulting in financial or other damages, you might also consider contacting an attorney as well, who may be able to work with law enforcement and send a cease and desist letter/court order, and file legal suit against the other party.