Now for the hard stuff.
If your Mac does something funny—or nothing at all—when you try to start it, first check for anything that might be wrong: stuck keys on the keyboard, stuck mouse buttons, cables that aren’t plugged in all the way, that sort of thing.
The First Law of Computer Repair is all-important: It’s always a cable until you have proved it can’t be.
Once you prove it’s not a cable, try booting from your Mac OS X installation CD and see if that works. To do this, hold down the C key while your machine attempts to start up. If it starts up, look to see if the hard drive appears on your desktop, and if all looks good, visit the Startup Disk preferences panel and see what ...