Can you tell me the exact version of OS X you are currently running, please?
Click the Apple logo in the menu bar at the top left of the screen
Select "About This Mac"
In the window that opens, look for the "Version" information. Please let me know what it says.
You are very welcome!
If you have this problem in the future, you may be able to quickly solve it as follows:
Restart your Mac
As soon as you hear the Startup chime, press and hold the SHIFT key
Continue to hold the Shift key until you see the Apple logo and progress indicator.
Then, let go of the Shift key and let it boot to the Desktop (it may take longer than usual so please be patient).
Finally, restart the mac once more as you normally would and your problem should be resolved.