There are four general causes of this issue:
1. The computer's PRAM no longer contains a valid startup disk setting when there aren't any problems with the disk itself. This can be checked for by pressing the Option key and seeing if the drive appears.
2. The internal drive's directory structure has become damaged. This requires usage of an alternate bootable system to perform the repair.
3. Critical system files have been deleted. This requires usage of an alternate bootable system to reinstall them.
4. The internal drive has died or become unplugged. This is the most likely case if the computer took a sharp impact or there are unusual sounds coming from the hard drive's location.
That means the startup disk is not readable for some reason...try starting in safe mode by holding the shift key during startup. If that fails to startup the Mac, insert your Snow Leopard install disk and restart holding the C key. When you get started, go to Utilities and Disk Utility and run repair on the internal hard drive to try and fix any errors in the file systems.
If that is successful, restart normally and see if the Mac starts.
If not it may then be necessary to reboot on the install disk and reinstall the operating system.
You need to re install OSX. If you have the dmg file downloaded via torrentz this is how you can create a bootable external HD or Flash drive (8 GB+)