What cause my MAC to crash? (Please restart computer now) The gray screen crash?

Answer 1

Testing and repairing your startup disk
If you have problems with your startup disk, you can use Disk Utility to test it for errors and repair it. To repair your disk, you must start up from another disk, such as your Install Mac OS X disc.

You may be able to test your startup disk without starting up from another disk. Open Disk Utility, select your startup disk, and click First Aid. If the Verify Disk button is available, click it to test your disk. You need to start up from another disk only if Disk Utility finds errors or if the Verify Disk button is dimmed. When testing your startup disk, Disk Utility may report errors when there are none. Starting up from another disk and then running Disk Utility gives more accurate results.

You can always test and repair disk permissions on your startup disk without starting up from another disk.

Start up your computer using another disk.
To use the Install Mac OS X disc, insert the disc and restart your computer holding down the Option key, then select the Install Mac OS X disc and click the arrow.

Open Disk Utility.
If you're using the Mac OS X Install disk, follow the onscreen instructions until the menu bar appears with the Utilities menu in it., and then choose Utilities > Open Disk Utility.

Select the startup disk in the list of disks and volumes, then click First Aid.
Check the S.M.A.R.T. Status at the bottom of the window. If you can't see it, be sure you selected the hard disk your volume is on, and not the volume itself.
If the S.M.A.R.T. Status is "About to Fail," back up your files on the disk as soon as possible and replace the disk.
If the S.M.A.R.T. Status is "Verified" or "Not supported," click Repair Disk to repair the disk.
If Disk Utility tells you to look for links to corrupt files in the DamagedFiles directory, two or more files occupy the same space on your hard disk and at least one of them is likely to be corrupt. Examine each affected file in the DamagedFiles folder, which at the top-level of the affected disk. If you can replace it or recreate the file, delete it. If it contains necessary information, open it and examine its data to make sure it has not been corrupted.

If Disk Utility cannot repair your disk or reports "The underlying task reported failure," try to repair the disk again. If that doesn't work, back up as much of your data as possible, reformat your disk, reinstall Mac OS X, and restore your backed up data. If you continue to have problems with your disk, it may be physically damaged and need to be replaced. See an authorized Apple dealer for more information.

Answer 2

Tutorial: An introduction to reading Mac OS X crash reports
http://www.macfixit.com/article.php?story=20060309075929717

Twenty steps to help diagnose and fix system issues
http://www.macosxhints.com/article.php?story=2004011205473937

Troubleshooting Mac OS X
http://thexlab.com/faqs/faqs.html