The screen is half blue

You many need to reseat the Apple DVI to ADC Adapter that is connecting your Apple Cinema HD 23" to your Mac.

If that adapter is not properly seated, it can cause a color distortion with the display. Also, if the adapter is damaged, that can also impact proper resolution and color balance.

To try and resolve this issue for you, I have a procedure I would like you to try:

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-SAFE BOOT MODE-

I am going to have you take your Mac into Safe Mode to see if we can't isolate what may be affecting your display's resolution.

Try these steps:

• Reboot your Mac
• Upon restarting your Mac, press the Shift key.
• Hold the Shift key until you see a White Screen with Gray Apple icon and eventually a spinning gear.
• When you see the Spinning gear, release the Shift key.
• Allow the Mac to continue booting up. Also this may take a little while
(NOTE: this may take anywhere from 5 to 15 minutes to load to the Safe mode screen depending on the performance of your Mac, but if it exceeds this time, shutdown your Mac and try just once more. It may have just frozen up the first time around for you.)

• You are going to enter the Safe Mode (which looks like your main desktop environment - the only difference is certain startup items are disabled.)

• Once within the Desktop in Safe Mode
• From the Finder Menu, click Go> Go to Folder
• Type in this line: /System/Library/StartupItems
• This path, is going to take you to the System Library folder which has all of your Mac OS X Startupitems.
• Remove all of those folders to the Trash.

*Reboot your Mac normally and see if you can access your normal User desktop environment.

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Here is an additional support reference for troubleshooting Apple Cinema Displays color or distortion issues.
Apple Support Link:
Calibrating your Apple Cinema HD Display for color accuracy
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This should help. If it does not let me know.

Regards,

-Mactechtrainer

Have you moved the DVI cables in the video card of the Mac ? Or if its attached to a pre DVI mac then the DVI adapter needs to be checked. So try moving the DVI connector in the plug and seeing if it has any effect. Check the pins from the end of the cables for damage.

Is the cable firmly attached? If so, the display is probably bad.