Try not to. If it still happens, go to the PKU campus cards office, tell them your Student No. and bring RMB 40 along for the processing fee. They will reissue you the PKU campus card (xiaoyuanka 校园卡).
If you don’t remember your student, inform the ECCS office. We can provide you with the number.
PKU Campus Card Office (Xiaoyuanka bangongshi 校园卡办公室): In the South-East corner of the No 1. Like Bldg. (理科一号楼) (map).