ORA-12514: TNS:listener does not currently know of service requested in connect descriptor / Fatal NI connect error 12514, connecting to /ns main err code: 12564 / TNS-12564: TNS:connection refused

Error like

Fatal NI connect error 12514, connecting to:
 (DESCRIPTION=(ADDRESS_LIST=(ADDRESS=(PROTOCOL=TCP)(HOST=server1)(PORT=25000)))(CONNECT_DATA=(SERVICE_NAME=DBNAME1)(CID=(PROGRAM=oracle)(HOST=server1)(USER=))))

  VERSION INFORMATION:
        TNS for HPUX: Version 11.2.0.3.0 - Production
        Oracle Bequeath NT Protocol Adapter for HPUX: Version 11.2.0.3.0 - Production
        TCP/IP NT Protocol Adapter for HPUX: Version 11.2.0.3.0 - Production
  Time: 17-JUN-2013 11:08:30
  Tracing not turned on.
  Tns error struct:
    ns main err code: 12564

TNS-12564: TNS:connection refused
    ns secondary err code: 0
    nt main err code: 0
    nt secondary err code: 0
    nt OS err code: 0

Possible Causes :

1.) The listener is down.
2.)The listener received a request to establish a connection to a database or other service. The connect descriptor received by the listener specified a service name for a service (usually a database service) that either has not yet dynamically registered with the listener or has not been statically configured for the listener.
3.)  This may be a temporary condition such as after the listener has started, but before the database instance has registered with the listener.

Solution.:

1.) Check your listner by issueing "lsnrctl status" command. 
Verify if the listener is up and running. 
2.) Verify your alert log and sqlnet.log to see if any errors.
3.) Verify if you have specified SID or SERVICE_NAME in you Global tnsnames.ora file.
Correct it if not set properly.