Try the following:

1. Restart holding down CMD+S
2. This brings up single-user mode, a UNIX command line.
3. Type fsck -fy and press return.
4. The OS will run repairs on some internal permissions and catalog files.
When it is finished type boot and press return.

THEN it should boot. If so, go via Safari to VersionTracker.com and download a program called Applejack and install it.

Once it is installed you can do a wider range of repairs on the system by starting with CMD+S to enter single-user mode and typing "applejack" and return.


A