Try resetting the PRAM.
1. Shut down the computer, hold the power button for 15 seconds.2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need them in step 4.3. Turn on the computer.4. Press and hold the Command-Option-P-R keys before the gray screen appears.5. Hold the keys down until the computer restarts.6. Release the keys and immediately and hold the Shift key until you see the Apple logo. This will activate Safe boot and it will check the drive for errors among other things. It may take longer to boot and you may see a progress bar during part of the process. If it starts up OK… login then restart the normal way from the Apple Menu. Let me know after.