Mac startup stops at a folder or disk with a blinking question mark.

If a Macintosh fails to start, displaying a small folder or disk icon containing a blinking question mark, there are several likely causes:

1. The hard drive crashed.
2. No startup disk was selected.
3. The directory structure on the disk containing the Mac OS has become corrupted.

(1) Hard disk crashes can be diagnosed by repeated clicking noises that are quite loud, consistent with a broken seek arm. A way to confirm the diagnosis is to start the computer off of disk repair CDs likeNorton Utilities, Techtools Pro or DiskWarrior. Hardware problems would prevent the disk from being seen by one of those disk repair utilities.

(2) If there is no startup disk selected (which may happen if you have just installed a new version of the operating system and have not selected which to boot from), you will need to boot from the CD-ROM and modify the Startup Disk control panel. Alternately, restart the computer and hold down the OPTION key. This should bring up a screen where you will be prompted to select a startup volume/folder.

(3) Some corrupt disk structures can be repaired using disk repair utilities like Norton Utilities,Techtools Pro or DiskWarrior. Refer to the documentation which comes with those programs on how to use them to repair disk problems.

More details about this error can be found here. The screenshots are for Mac OS 9, but this problem can occur in OS X as well.

Note: Symantec has phased out support for Norton Utilities and it will not work in Mac OS 10.4.