IMAC wont start
Imac wont start has gray screen with apple logo or world globe logo and progress bar shuts down
iMac, OS X Mountain Lion (10.8.2)
See if you can do this...
Bootup holding CMD+r, or the Option/alt key to boot from the Restore partition & use Disk Utility from there to Repair the Disk, then Repair Permissions.
Or hold the alt key at bootup to see if you can choose the AHT & run the extended Hardware test.