1. PRAM reset
Reboot, after mac has shutdown, immediately, hold down:
[Option]+[Command]+{P}+{R}
after a few moment mac will reboot again, then release those keys.
2. Safe boot, will clear out the cache stored on the mac hard drive
Reboot mac,
hold down [Shift] key during the boot process until you reach the login screen.
if you did it right, you can see the words Safe boot in the login pane.
Login and reboot again.
