ORA-12500

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

For Above error there is couple of reason check below point.

1.Oracle Services is start.
Windows > c:\>net start OracleService
Unix > $ps -ef | grep ora_smon

2.Listener Services is start.

3.Take a look on LISTENER.LOG file {%ORACLE_HOME%/network/log}

4.if you received WINEXEC FORMAT ERROR 8

C:\>net helpmsg 8

Not enough storage is available to process this command.
Contact OS vendor for more information.

5.Sometimes it is happen due to PGA RAM shortage on database.
You have to set below parameters.
pga_aggregate_target, sort_area_size and hash_area_size.

6.TNS-00517: Lost contact
32-bit Windows Error: 54: Unknown error
If your received above error
C:\>net helpmsg 54

The network is busy.

7.TNS-00507: Connection closed
32-bit Windows Error: 109: Unknown error

C:\>net helpmsg 109

The pipe has been ended.

8.Recreate OracleService service through ORADIM utility

ORADIM -DELETE -SID
ORADIM -NEW -SID

HOPE THIS HELPS
Taj