End user's session was interrupted (maybe TCPIP went down). When the user tried to log back in, he/she got message: KC06 User XXXX is already logged on at terminal XXNN.
To get the user logged back on, from the main menu: go into SECURITY, EDIT, select the user, DEFINE. Then change the parameter "USER MAY LOGON AT MULTIPLE TERMINALS" from NO (the default) to YES. That will allow the user to log back in.
When CICS is recycled, the locked-up user session will be freed.