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Lets try this repair first
Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
Turn on your Mac.
Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
Hold these keys until the computer restarts and you hear the startup sound for a second time.
Release the keys.
Then, hold down the left shift key as soon as your mac starts to turn on.. Keep holding it till you see the apple symbol. This should attempt to start the machine in safe mode.
If it wont start after ten minutes send a reply.. If it does start in safe mode.. Run this repair next and then get back to me.
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 .
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Senior Technician and Network Switching Systems Operator / Maintainer.