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While Apple computers and the Mac OS X operating system are renowned for their reliability and ease of use, it is still possible for issues to occur that will make your iMac system unresponsive to input. The iMac may have frozen due to a software program causing an error with the operating system or due to a hardware malfunction. Fortunately, there are several steps you can take to correct any resultant problems and unfreeze your iMac for further use.
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Use "Force Quit" to Unfreeze your iMac
If your Apple iMac has frozen up while you are using a specific piece of software, the issue probably lies with the software. It may have made an incorrect demand upon the OS X operating system, consuming all its resources, or the software might be installed incorrectly. This type of freezing error will usually be signified by a "Spinning Beachball" icon replacing the mouse cursor. The "Force Quit" dialog box allows a Mac user to halt a program even if the interface is unresponsive.
To call up the "Force Quit" dialog, either select it from the Apple menu at the top left of your iMac's Menu Bar, or if you have no mouse input hold the "Cmd", "Option (Alt)" and "Esc" keys. When the "Force Quit" dialog box appears, select the software program you were using and choose "Force Quit". Very occasionally, you may have to do this more than once to get the unresponsive program to stop.
After your iMac has unfrozen, open the offending program again, and see if the error occurs again. If it does, the software installation itself may need repairing. To do this, open "Disk Utility" and select your hard drive (usually labelled "Macintosh HD"), then press "Repair Disk Permissions". The program will then search for and attempt to repair any incorrect software permissions.
If this does not stop the freezing problem on your iMac when using a particular piece of software, you may need to completely reinstall the offending software.
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Unfreezing a Totally Unresponsive Apple iMac
If your iMac is completely frozen, this can indicate a more serious issue with the operating system or hardware. If "Force Quit" does not help, and you are unable to use the "Shut Down" option in the Apple Menu, you will have to manually shut down your iMac using the physical Power Button. Hold the Power Button for a few seconds until the computer shuts down.
Restart your iMac and see if Mac OS X boots up normally. If your iMac starts as normal then the problem may only have been transitory, although you should keep an eye out for any other unusual behavior or long delays when using the computer in future.
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Starting an iMac in Safe Mode
If the problem persists but you do have access to some of the functions of your iMac, you can troubleshoot your iMac by starting it in "Safe Mode". To do this, press the Power Button, and then hold the "Shift" key when you hear the familiar startup chime. You can let go of the "Shift" key once the Apple logo and grey background appear. Now go to the "Accounts" section in the "System Preferences" dialog box and choose "Login Items". In this section are listed applications that launch when Mac OS X starts, and may be causing conflicts with the operating system. Uncheck them all, and then restart in "Safe Mode" again and observe for any issues. If there are none, return to the "Login Items" and tick one application at a time, rebooting after each one to narrow the culprit down. Once you have found the offending application, repair permissions or reinstall the software.
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Unfreezing an iMac that Won't Start OS X
With some serious freezing problems, you may not be able to start the Mac OS X operating system at all, even if "Safe Mode". If this happens, find the OS X disc that came with your iMac, insert it into the disc drive and restart while holding down the "C" key. This will make the iMac boot from the copy of OS X on the disc. Choose a language, and then select "Utilities" from the Menu Bar when the Installer screen appears. Now choose "Disk Utility". Select the hard drive of your iMac and press "Repair Disk". This can be quite time consuming, but it is important to wait till the program has finished its work. When the repair is completed, you can restart the iMac as usual. Check for any further issues or freezing.
If there is a fault with the iMac hard drive itself, then Disk Utility will inform you that it is cannot be repaired. This can happen when an iMac has suffered a hard knock or other movement while OS X is writing data, and actual sectors of the hard drive have been damaged. Unfortunately, this problem can only be solved by fitting a new hard drive and reinstalling Mac OS X onto it. If your iMac has suffered this serious problem, you should contact Apple Support for the nearest Technical Repair provider to you.
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The Apple iMac is a very stable and reliable computer, and problems that can result in the computer freezing are fortunately very rare. If it becomes necessary to unfreeze an Apple iMac, following the above steps will help to identify and solve the freezing issue.