Administrator Password pop up screen does not open I have Macpro4.1 running Yosemite. Suddenly I can't do anything that requires Administrator's password. The pop up screen to type in password does not come up.
If I go to Users &amp; Groups and click on the lock, It will say authenticating but just spins and password screen does not open. If I try to access passwords in Keychain the app will lock up and I have to force quit.
Any ideas on how to fix this problem?
I would suggest you to: 

Reset NVRAM or PRAM on your Mac

Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. You can release the keys after about 20 seconds, during which your Mac might appear to restart.



Test after this. 
Note: When your Mac finishes starting up, you might want to open System Preferences and adjust any settings that were reset, such as sound volume, display resolution, startup disk selection, or time zone.

If this doesn't help then at last try to: 
Reset the SMC on Mac

First, shut down your Mac and Press Shift-Control-Option on the left side of the built-in keyboard, then press the power button at the same time. Hold these keys and the power button for 10 seconds. Release all keys. And, Press the power button again to turn on your Mac.

This two reset should help.