This means your Mac cannot see your operating system on your hard drive.
This could be because
1. You hard drive has broken
2. Or even that the cable between your hard drive and the circuit boards inside your Mac have become loose (yes, happened to me on a G4 Tower once after moving it around) or broken.
If you can open your Mac (if it is a tower) check the cables first.
Then reboot.
If still nothing:
Can you hear your hard drive spinning?
Is it clicking?
Next step, put in original Mac OS X install disc and restart Mac holding down the C key.
You will then be runing your Mac from the operating system on the install disc.
Go to the menu bar and selct Utilities.
Then Disc Utilities.
Can you see your hard drive in the left panel?
If so, do a repair disc on it.
If you cannot repair the disc, you will have to do a erase and install from the OS disc. You WILL LOSE ALL YOUR DATA. But you will have your Mac back.
Get back to us and tell us what you see.