Improper shutdown or power interruptions can create file system errors that stunt the start-up process. This article provides a couple preferred ways to fix these issues on a Mac computer.
If using Mac OS X 10.2 or later, you can start up in Safe Mode, which automatically performs disk check and repair:
Make sure your computer is shut down – then when turning the computer back on, start holding the shift key immediately after you hear the start-up tone. Release the key when you see the gray Apple icon and spinning wheel. A progress bar should then appear, indicating that safe mode is performing a directory check. Safe mode should take itself though any checks and repairs, and you can restart your computer without holding any keys to leave safe mode.
Note: Safe Mode won’t work if you have a firmware password.
If running Safe Mode is unsuccessful in fixing start up issues, or if you’re using a version earlier than Mac OS X 10.2, fix file system issues using Disk Utility:
Boot into the Recovery System – press command+R as early as possible during start-up, and hold until the Apple icon appears. (Note: you can also access Disk Utility from Mac OS X Internet Recovery – for more information, click on the Apple link at the bottom of the page.) Following Recovery System start-up you should see a desktop with an open “Mac OS X Utilities” window. Choose “Disk Utility” from this window.
If running Mac OS X v10.6 or earlier, you’ll need to run Disk Utility from a Mac OS X install or restore disc. Hold the C key during start-up after inserting the disc. Choose Disk Utility from the Installer menu. (Do not click Continue in the first screen of the installer, or you’ll have to start up from the disc all over again to access Disk Utility.)
Once in Disk Utility, either from an installation disc or in the Recovery System, proceed to the “First Aid” tab. Click the triangle to the left of the hard drive icon to display a list of your disk volumes and partitions. Select your OS X volume, and click “Repair”. Disk Utility will then run a scan and repair on the disk.
If you’re still experiencing issues starting up your computer, there are some alternatives to try, including use of fsck in a command-line interface. For instruction on this, and for other possible methods of repairing system files, check out Apple’s site.