If your Macintosh is behaving erratically or is not booting up all the way, you may want to try resetting the PRAM. You can reset or "zap" the PRAM on a Mac by pressing and holding the Command, Option, P, and R keys right as you turn the computer on. Yes, there are four keys you need to hold down at the same time. Once your hear the startup chime a second time, the PRAM has been reset and you can release the keys.
Note that if you choose to zap your PRAM, you may need to set your display, time zone, startup volume, and other affected settings using System Preferences. Some firmware updates may reset your PRAM as part of their installation process, which will means you will have to reset the affected settings after the installation.