The database is down. Please specify the host credentials to access database restart and diagnostics tools

The database is down. Please specify the host credentials to access database restart and diagnostics tools.
Enterprise manager shows that database is down but in real it is not.

[oracle@oel6 trace]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.3.0 Production on Fri Sep 13 10:38:01 2013

Copyright (c) 1982, 2011, Oracle.  All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, Automatic Storage Management, OLAP, Data Mining
and Real Application Testing options

SQL> select status from v$instance;

STATUS
------------
OPEN

check status of listener

[oracle@oel6 log]$ lsnrctl status

LSNRCTL for Linux: Version 11.2.0.3.0 - Production on 16-DEC-2013 11:54:14

Copyright (c) 1991, 2011, Oracle.  All rights reserved.

Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for Linux: Version 11.2.0.3.0 - Production
Start Date                16-DEC-2013 11:51:59
Uptime                    0 days 0 hr. 2 min. 15 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Log File         /u0/app/oracle/diag/tnslsnr/oel6/listener/alert/log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=oel6)(PORT=1521)))
Services Summary...
Service "+ASM" has 1 instance(s).
  Instance "+ASM", status READY, has 1 handler(s) for this service...
Service "orcl" has 1 instance(s).
  Instance "orcl", status READY, has 1 handler(s) for this service...
Service "orclXDB" has 1 instance(s).
  Instance "orcl", status READY, has 1 handler(s) for this service...
The command completed successfully

check status of dbconsole

[oracle@oel6 oracle]$ emctl status dbconsole
Oracle Enterprise Manager 11g Database Control Release 11.2.0.3.0 
Copyright (c) 1996, 2011 Oracle Corporation.  All rights reserved.
https://oel6:1158/em/console/aboutApplication
Oracle Enterprise Manager 11g is running. 
------------------------------------------------------------------
Logs are generated in directory /u0/app/oracle/product/11.2.0/dbhome_1/oel6_orcl/sysman/log 

it's runing, after that i checked alert.log but there was nothing about some errors.
Solution:
select username, account_status from dba_users order by 2;
shows that
DBSNMP EXPIRED
SYSMAN EXPIRED
SYSTEM EXPIRED(GRACE)

now simple change their statuses.

alter user SYSTEM identified by oracle;
alter user SYSMAN identified by oracle;
alter user DBSNMP identified by oracle;

if they are locked then simple unlock with commands:

alter user DBSNMP account unlock;
alter user SYSTEM account unlock;
alter user SYSMAN account unlock;

it's all, now enjoy with enterprise manager