This means that your machine cannot find a valid OS install. There are sever reasons why. Step one would be to boot to your install disk and verify and repair permissions and the disk if necessary. Try again. Now if it still doesn't work then you could try an archive and install. If this still fails then DiskWarrior would be handy.
It could be that the disk is damaged in some way, or the connections loose, but from what you have said I wouldn't imagine this to be the case. I'd start with the above and go from there.