First, of course, try to recall when you last used your cell phone and where you might have lost it or where it might have been stolen. Go through the last places you’ve been and maybe ask people (i.e. in a café, restaurant etc.). If you think you left it in a taxi, contact the taxi company immediately and tell them the route you took and the individual taxi number, which you find on the upper right of your taxi receipt. Therefore, always get a receipt from your taxi driver! Without the receipt it is impossible to find your cab among a total of some 80.000 taxis in Beijing.
If your cell phone is gone for good, at least your old number and balance can be retrieved. In order to do so, take the card with the PIN number you got when you bought your SIM card and go to the next China Mobile or China Unicom store. They will deactivate your old SIM card and transfer your number and balance to a new one (against a small fee (¥10). If you can’t find your card with the PIN number you can probably still get you old number and balance back by answering a few control questions.
China Mobile office: 中国移动通信 Zhongguo Yidong Tongxin (map), just opposite of the PKU South Gate.
China Unicom office: 中国联通 (map), on campus in the PKU telephone office.