ORA-12500: TNS:listener failed to start a dedicated server process

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sqlplus usr/pwd@orcl

ORA-12500: TNS:listener failed to start a dedicated server process

This error can be seen on Windows platforms when the file LISTENER.ORA located in the directory %ORACLE_HOME%\network\admin contains an explicit database service declaration (see below in green) and the database has not been started:

...
SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
 (SID_NAME = PLSExtProc)
 (ORACLE_HOME = C:\oracle\product\10.1.0\Db_1)
 (PROGRAM = extproc)
)
 (SID_DESC =
 (GLOBAL_DBNAME = orcl)
  (ORACLE_HOME = C:\oracle\product\10.1.0\Db_1)
  (SID_NAME = orcl)
 )
)
...

This means that you should contact the DBA and ask why the database service is down.

A misconfiguration of the TNSNAMES.ora file would lead to different errors, like ORA-12541 (when TCP PORT numbers mismatch for instance).
Note that you can remove the explicit declaration of the database service from the LISTENER.ora file in case you are using a standard listener configuration (on port 1521) because the database should be able to register itself after starting up. If something goes wrong in this case, you may get ORA-12514 or ORA-12505, most likely.

See message translations for ORA-12500 and search additional resources