STEP 1. First you have to restart your mac then before you get to the ‘Welcome to Macintosh’ screen simply you have to hold COMMAND-OPTION-’P’-’R’ key combination from your Keyboard.
STEP 2. Now you will be able to hear the startup chime, keep holding the keys until you heard three chimes.
STEP 3. Now the PRAM of your Mac will be reset to the default values.
That’s it. In this way you can reset the PRAM of your Mac. You should note that during this process your customized settings might be lost. In order to restore your customized settings you can use the Control Panels