How to Reset the PRAM on a Mac
There is a section of auxiliary memory that the Mac's operating system reserves for a few important functions such as which disk to use when the computer starts up and your display parameters. Called the parameter random access memory, or PRAM, resetting it may clear up problems if you are having unexplained issues with parts of your operating system. Hooking up a new display, for instance, may cause a problem that a PRAM reset could solve. After the reset, you may need to change some settings, like for the clock or time zone, if you had altered them manually prior to the reset.
Copy or print out these instructions.
Turn off your Mac.
Turn the Mac back on, and immediately hold down the "P," "R," "Option" and "Command" keys. You may want to find the keys and figure out which fingers to use prior to executing this step.
Continue depressing the keys until the second start-up chime, then release them. You may now operate the Mac normally.