ORA-12523: TNS:listener could not find instance appropriate for the client connection

ORA-12523: TNS:listener could not find instance appropriate for the client connection
 *Cause:  The listener could not find any available (database) instances,
 that are appropriate for the client connection.
 *Action: Run "lsnrctl services" to ensure that the instance(s) are
 registered with the listener, and have status READY.

C:\Oracle\ora102\NETWORK\ADMIN>type tnsnames.ora
...
MIKA0 =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = tcp)(HOST = FR9050482D)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SID = MIKA)
    )
  )
...
C:\Oracle\ora102\NETWORK\ADMIN>sqlplus /@mika0

SQL*Plus: Release 10.2.0.3.0 - Production on Tue Jan 29 14:00:55 2008

Copyright (c) 1982, 2006, Oracle.  All Rights Reserved.


Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Production
With the Partitioning, Oracle Label Security, OLAP and Data Mining options

SQL> select server from v$session where sid=sys_context('userenv','sid');
SERVER
---------
SHARED

1 row selected.