Something horribly went wrong either with the hardware or OS X. So did you use the original OS Install disks that came with the computer because one of those has "Hardware Test" written on it. You can boot to the Hardware tests and test to make sure the hardware didn't fail after such a long use. Plus if you have the OS X install disk you can boot with that and Use Disk Utility and test to see if the hard drive failed.
There can be server things that could be wrong. So have you tested to see if the hard drive has failed?