This simply means that it can't find the System Files to bootup from, perhaps you may want to bootup from the OS CD and make repairs to the drive using the Disk Utility
You should also try rebooting into Single User Mode (cmd-s on reboot) and run fsck (type "fsck -f" at the prompt, then "reboot" when the disk repair is completed). If that doesn't work, you're probably in for an archive and install.
Go into system preferences. Go to startup disk. Select the proper disk to start up from. Then restart and see if you still get the same icon at startup.
I had this problem with a Emac. I finally gave up and went to do a archive and install, but it could not find the hard drive. Luckly apple care replaced the hard drive for free!