应尽快向中国当地公安机关报失，并到美国驻华使领馆申请新护照或其他国际旅行证件，再到中国公安机关补办停留证件或出境手续。　

You should report the loss to the local Chinese public security organs as soon as possible, and apply for a new passport or other international travel documents at the U.S. Embassy or Consulate in China, and then go to the Chinese public security organs to reapply for the stay permit or exit procedures.