Basically nothing can be done. Some people believe that they can put a 'tracer' on lost mail, and that is true, but doesn't guarantee the retrieval of the lost item. All a 'tracer' does is ask the sender if they sent it and the receiver if they received it. You apparently already know that, so the tracer is moot point.
If the item was Registered or Certified, there are other methods for locating as each time the letter is handed off to another person, a signature is required.