Your computer is having trouble locating your os x installation.
I'd boot in Safe Mode, so a repair and try to restart:
1. Restart, hold command + s till the black screen with white text shows up
2. type this EXACTLY without the quotes: "fsck -f" and wait for process to complete. You'll see the cursor jump down to a new line when complete.
3. type this EXACTLY without the quotes: "reboot"
If this doesn't work, you'll need to boot up on your os x system install disk 1 but DON'T REINSTALL OS X. Go to File/Disk Utility and "Repair Disk".