Mission control and Cmd + Tab not working since Yosemite 10.10.3 I upgraded my OS from Mountain Lion to Yosemite yesterday and now Mission Control and Cmd + Tab aren't working anymore. Clicking mission control icon doesn't do anything, although I still have 4 spaces automatically opened upon starting the computer. Also, for instance, when I am on screen #1 and Chrome is opened on screen #2, if I press Cmd + Tab to change to Chrome, the menu bar at the top of the screen will change to Chrome but the screen will remain on screen #1.

So far, I tried, but without success:


Rebooting
Killing dock process using "killall Dock" and "killall -kill Dock" in Terminal
I made sure no other updates were available
Browsed internet a few hours to try to find something, but found nothing successful
F4 shortcut doesn't work neither


My computer specs are the following:

iMac 27 inches, late 2009,
2.66 GHz Intel Core i5,
8 GB DDR3

Would anyone have any suggestions?
Try resetting all the defaults for Mission Control. I had the same problem and now it works. 

Type the following command into Terminal:

defaults delete com.apple.dock mcx-expose-disabled &amp;&amp; killall Dock