Start by contacting your local postmaster to request a copy of the address change order and instructions on how to revoke it. Then, with the information regarding the address to which your mail was diverted, contact law enforcement.
Best wishes for a favorable outcome, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.
The best police authority for your problem is the US Postal authority itself.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.