From time to time, even well maintained computers will experience operating bugs. A common glitch for iMac users is a "frozen" screen. When the computer freezes, one or more applications are stuck in a certain place on the screen, unable to be modified or closed. In other words, the program you're working on simply locks up. In other cases, computers can get stuck on certain start-up operations, freezing the entire screen image. Both common freezes can be corrected with the right user actions.
Press "Command," "Escape," and "Option" simultaneously on your keyboard. This will open up the "Force Quit" window.
Click on the name of the frozen program from the list of applications listed under the "Force Quit" menu. Typically, the problematic program will be listed first by default.
Click "Force Quit" once the program is selected. This will shut down the frozen program, returning your frozen screen to a normal, functional state. Just remember, you've only unfrozen the screen, you haven't fixed the underlying error. The program may still freeze again if you try to restart it. To correct the problem, you may want to reinstall the program.
Wait at least one minute to confirm that the screen is truly frozen. Your iMac may just be running sluggishly, making it appear frozen for several seconds during start-up.
Press your iMac power button for about 5 seconds. According to the iMac user manual, this will trigger a shutdown of your system (see Reference 1). If your computer is unable to boot up, the only way to unfreeze the system is to completely restart it. By holding down the power button, the computer has time to dump memory operations before shutting down. This is a safer shutdown process for your computer than simply unplugging the computer from the wall.
Press the power button again to start the computer again. If the frozen screen was just a minor glitch, the computer should reboot normally. If the computer continually freezes on start-up, you may need to reinstall your iMac operating software from the backup disc that came with your computer. To do this, you simply insert the disc labelled "Max OS X Install DVD" during reboot and follow the on-screen prompts.
Only reinstall your operating software as a last resort, as this can delete files on your computer.