This may happen if the computer starts up from the hard drive or from an external drive using the wrong version or build of the operating system.
Reinstall the correct operating system build by using Mac OS X Install DVDs included with the computer. You can also use a later build, but never install a version of the operating system released before the computer.
Learn about restoring the correct operating system version using a restore DVD or Time Machine backup.
To confirm that the computer is attempting to start up from the wrong operating system version, start up the computer in verbose mode:
- Shut down the computer if it is on.
- Press the power button to start the computer.
- Immediately press and hold the Command (Apple) key and the "V" key (Command-V) for verbose mode.
Verbose mode is on if you see white text appear on the screen as in the image below.
If the computer has the wrong build of the OS installed, the startup process will stop. The following line of text will appear on the display: "Loading System\Library\Caches\com.apple.kext.caches\startup\Extensions.mkext."
Exit verbose mode by pressing the power button until the computer shuts down.