Try these one at a time, see if it is any help:
- Shut down your Mac.
- Locate the following keys on the keyboard: Option, Command (⌘), P, and R. You will need to hold these keys down simultaneously in step 4.
- Turn on your Mac.
- Immediately press and hold the Option-Command-P-R keys. You must press this key combination before the gray screen appears. Continue holding the keys down until your Mac restarts, and you hear the startup sound for the second time.
- Release the keys.
- Press Command + Option + Escape to force quit any application that is not responding.
- Put your Mac to sleep by choosing the Apple () menu from the upper-left menu bar and then choosing Sleep. Wake the computer after it has gone to sleep.
- Restart your Mac by choosing the Apple menu from the upper-left menu bar and then choosing Restart.
- Shut down your Mac by choosing the Apple menu from the upper-left menu bar and then choosing Shut Down.
Applications folder/Utilities folder (or from FINDER click GO in the menu bar and select Utilities or from Finder SHIFT+COMMAND+U) Disk Utility, select (highlight) your volume from the left sidebar (default name Macintosh HD) Click repair permissions.