Stuck in restart - how to get out and reboot

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Try powering off and restarting in 'Safe Mode': The instant you hear the startup tone, hold down the shift key and keep it held down as explained in this Apple support note:

http://support.apple.com/kb/ht1455

A Safe Mode restart does take much longer than a normal start, because the computer checks the hard drive and ignores cached kernel extensions that normally speed boot-up.

http://support.apple.com/kb/TS1884

Note that not all devices and services, notably WiFi, may be available after a Safe Mode start.

This is not necessarily an indication of a hardware error.

After your computer starts in Safe Mode, you should shut down, and attempt to restart normally.