Have you tried resetting the PRAM? When you turn it on, hold down Apple+Option+P+R and keep holding it until after it restarts and you hear it do the startup sound for the second time.
Is this a PowerMac or an iMac? If it is a PowerMac, it wouldn't happen to be the Quad 2.5GHz model would it? If it is, then you may have suffered a catastrophic loss of coolant accident, and I would suggest pulling the side off the case and looking for any discoloration in there.
If it is not the Quad 2.5, then that can not happen, and I am not sure what else to tell you.
Try dropping your Install disk in the drive, and see if it will load that up (hold C when booting). If it does then it is possible that your hard drive failed.