It can mean a variety of things, but mainly it's because the boot up permissions file has become corrupted or the motherboard is toast. To remedy problem one, use the OS installation CD and a run an archive and install. If that doesn't do it, it's probablt the motherboard.
Tried running the OS disk but it couldn't find the drive so I opened the disk utility and no drive could be found which means the drive is dead as I change it to a regular drive it was read the drive but still wasn't able to load the OS.
Q. Should I try to find a more compatible drive to MAC or should any 2.5 HDD work??