During the events of the G.U. games the system anomaly, AIDA, was infecting many players and causing them to fall into comas in the real world. These people are referred to as the "Lost Ones".
A prime example of a Lost One would be Shino, after her encounter with Tri-Edge at the Hulle Granz Catherdral. Another good example of a Lost One would be Alkaid after she is PKed by the AIDA-infected Bordeaux.