(PRAM/NVRAM)
Finally, you can perform two system adjustments that help some mysterious problems:
Try to reset your PRAM:
1. Shut down the computer.
2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Press the power button to turn the computer on. You will hear the computer’s startup sound.
4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.
Also reset the NVRAM (Will not work with an Intel Mac):
First, shut down your computer. Then boot up, and press and hold the Command-Option-O-F keys. You must press this key combination before the gray screen appears.
Then, do the following
1. At the Open Firmware prompt, type: reset-nvram
2. Press Return.
3. At the Open Firmware prompt, type: set-defaults
4. Press Return.
5. At the Open Firmware prompt, type: reset-all
6. Press Return.
7. If your computer does not automatically restart, then type: mac-boot