1. Check all conection. and 2. Zap the PRAM(Restart, while screen is blank, hold down option, command, p and r until two start up chimes). *try 3. Put a system install disk in the slot loading cd-player as you hold down the option key, it might show a. icon of your HD and b. the system install disk. If it does click on the sys. install disk, and find the disk utilities to try to repair permissions etc. *try 4. Safe start, hold down the shift key when restarting. This turns off all extensions. *asterisks means not sure if you INSTALLED SYSTEM has these options. *Disk warrior and/Tech Tools are good disks to try to start-up on.