Very odd.
Can you boot up at all, or does it kernel panic/freeze every time?
Also, have you tried booting into Disk Utility? From the manual (PDF):
Repairing, Restoring, or Reinstalling Mac OS X Software
If you have a problem with your Mac software or hardware, Mac OS X provides repair and restore utilities that may eliminate problems and can even restore your software to its original factory settings. You can access these utilities from the Mac OS X Utilities application, even if your computer isn’t starting up correctly.
Use the Mac OS X Utilities application to:
• Restore your software and data from a Time Machine backup.
• Reinstall Mac OS X and Apple applications.
• Restore your computer to its factory settings by erasing its disk and reinstalling Mac OS X and your Apple applications.
• Repair your computer’s disk using Disk Utility.
If your computer detects a problem, it opens the Mac OS X Utilities application automatically. You can also open it manually by restarting your computer.
To open the Mac OS X Utilities application:
Restart your computer while holding down the Command (⌘) and R keys. The Mac OS X Utilities pane appears.
Some utilities in the Mac OS X Utilities application require access to the Internet and the Mac App Store. You may need to make sure your computer is connected to the Internet by using your Ethernet or Wi-Fi network.
To connect by using a Wi-Fi network:
Choose a network from the Wi-Fi status menu in the upper-right corner of the screen. If necessary, type the network password.
To join a closed network, choose Join Other Network. Enter the network’s name and password.
Repairing a Disk with Disk Utility
If you have a problem with your computer, or if you start your computer and see the Mac OS X Utilities application, you may need to repair your computer’s disk.
Select Disk Utility in the Mac OS X Utilities pane and click Continue.
Select the disk or partition in the list on the left, and then click the First Aid tab. Click Repair Disk.