Mission Control stopped working then Dock won't restart i am using a macbook air and with yosmite os

At first I could still switch between desktops, just that i could not swipe up/down to access mission control and app exposé. I tried typing in killall Dock into terminal to restart dock, but now i cant switch desktops either and the background is completely black. killall dock just tells me it can't find the process for this user. when i typed in killall -s dock it showed only one process.

is there a way to force dock to start and fix mission control?
This was happening to me for a long time and I did some research and found out how to fix it. Go into terminal and run the following commands.

defaults write com.apple.dock mcx-expose-disabled -bool FALSE
killall Dock


This should fix all of your problems.