On startup Oracle must read the control file in order to find out where the datafiles and online logs are located. Oracle expects to find control files at locations specified in the CONTROL_FILE initialisation parameter. The instance will fail to mount the database if any one of the control files are missing or corrupt. Here's an example:

SQL> startup

ORACLE instance started.

Total System Global Area 135338868 bytes

Fixed Size 453492 bytes

Variable Size 109051904 bytes

Database Buffers 25165824 bytes

Redo Buffers 667648 bytes

ORA-00205: error in identifying controlfile, check alert log for more info

SQL>

On checking the alert log, as suggested, we find the following:

ORA-00202: controlfile: 'e:\oracle_dup_dest\controlfile\ORCL\control02.ctl'

ORA-27046: file size is not a multiple of logical block size

OSD-04012: file size mismatch (OS 5447783)

The above corruption was introduced by manually editing the control file when the

database was closed.

The solution is simple, provided you have at least one uncorrupted control file - replace the corrupted control file with a copy using operating system commands. Remember to rename the copied file. The database should now start up without any problems.