Most of the time resetting the PRAM will solve this issue:
# Shut down your Mac.
# Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
# Hold Down the keys
# While holding the keys, press the power button. Technically you don’t need to press them until it turns on, but you need to beat the gray screen, so it will be easier for you to just hold them down from the start. Keep holding.
# Hold the keys down until the computer restarts and you hear the startup sound (Chime) for the second time.
# Release the keys. Your mac is going to start up normally, and most likely the problem will be solved.