By Mark L. Chambers from "Mac OS X Lion All-in-One For Dummies"
When your Mac OS X Lion system acts up, the best way to troubleshoot is to do the simple things first and explore more complicated solutions as needed. Of course, if you already know that you’re having a problem with one specific peripheral or one specific application, feel free to jump to the steps that concern only hardware or software.
1. Try a simple shutdown. You’d be amazed at how often a reboot (the process of shutting down and restarting) can cure a temporary problem.
If your Mac is locked tight and you can’t use the Shut Down command from the Apple menu try pressing and holding the Power button on your Mac for a few seconds to turn your computer off. If this doesn’t work — and, rarely, it doesn’t —physically pull the power cord.
2. Check all cable connections. Check the AC power cord and the keyboard cord, as well as any modem or network connections and all cable connections to external peripherals. If you’ve recently replaced a cable, replace it with a spare to see whether the problem still occurs.
3. Retrace your steps. Consider what you’ve done in the immediate past:
* If you added an external device: Turn off your Mac and disconnect the peripheral. Then turn on the computer to see whether all proceeds normally.
* If you installed new software or applied an update/patch: Uninstall the application and search for any files that it might have created elsewhere. (Searching by date created and date modified can help you locate files that were recently created.)
* If you recently made a change within System Preferences: It’s possible that you’ve inadvertently “bumped” something. Verify the settings screens that you visited to make sure that everything looks okay.
4. Run Disk Utility. This checks for disk errors and permissions errors — especially permissions errors, which can wreak absolute havoc on just about any application on your hard drive. (Click the Disk Utility icon in the Utilities folder inside your Applications folder.)
5. Run antivirus software. Scan your entire system for viruses, including all system disks and removable disks.
6. Check the Trash. Open the Trash window and check that you haven’t inadvertently tossed something important. If you have, drag it back to its proper folder.
7. Check your online connections. Check your equipment to make sure that you’re currently online and receiving packets normally.
8. Disable troublesome Login Items. Disable automatic logins from the Login Items settings in the System Preferences Accounts pane: Click the Apple menu, choose System Preferences, click Users & Groups, and then click the Login Items tab. Enable them again one at a time.
9. Turn off your screen saver. Deactivate the saver entirely or choose the Computer Name saver (provided by Apple) from the Screen Savers list.
10. Check for write protection. If you’re running a multiuser ship, check that another user with administrator access hasn’t accidentally write-protected your documents, your application, or its support files.
11. Check your System Information. If you haven’t found the culprit yet, you’ve probably experienced a hardware failure in your Macintosh. If possible, display the Hardware category within the Apple System Information utility and make sure that it can recognize and use all the internal drives, ports, and external devices on your Mac. To start System Information, open your Utilities folder (inside the Applications folder) and double-click the System Information icon.
12. Reboot with the Mac OS X Recovery HD volume. This is the last step that you can take before you seek professional assistance. Hold down the Option key immediately after you hear the startup chord (which displays a boot menu), choose the Recovery HD volume and then run Disk Utility from the window that appears. Because you’ve booted the system from the Recovery HD volume, you can verify and repair problems with your startup hard drive. (Some new Mac models also come with a diagnostic DVD that can help you pin down hardware problems.) After you’re done, restart your system.