Lets try this as a first step.
- Shut down the computer.
- Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
- Turn on the computer.
- Immediately press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Continue holding the keys down until the computer restarts, and you hear the startup sound for the second time.
- Release the keys.
Then, if the mac still wont start.. Try this next.
- Be sure your Mac is shut down.
- Press the power button.
- Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone, but not before the tone.
- Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).
If your mac will now start, please try this next.
Open your finder ( magnifying glass ) type in disk utility, and launch the disk utility application.
On the left side of the disk utility window, click your mac hd, then at the lower right hand side, click repair permissions .
Once it says repair permission complete.. Restart your mac, will it start normally now?
( if your mac wont start after the first two repair attempts, please send a reply and let me know so that you and I can continue working this issue together, as the first steps I suggested are definitely strong steps towards repairing the issue, but they wont work iin 100% of cases)
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Senior Technician and Network Switching Systems Operator / Maintainer.