Force Restart the MacBook Air.
Force restarting a Mac is generally considered a last resort, but if the worst happens and your machine freezes up, it's the quickest way of getting things running again.
Before taking this extreme option though, it's worth making sure that you've exhausted all potential software solutions.
If it's just an application that's frozen rather than the operating system, hold down the Option (⌥) key then right-click the offending app's icon in the Dock and select Force Quit.
If the OS is frozen but the mouse cursor still responds, click the Apple () symbol in the upper left corner of the screen and select Restart. Otherwise, try holding down the Control (Ctrl) key and pressing the power button. If the system shutdown dialog appears, select Restart from the available options.
To Force Restart, Press and hold down the Command (⌘) and Control (Ctrl) keys along with the power button until the screen goes blank and the machine restarts.
If this fails too, press and hold the power button until the screen goes blank and the machine restarts.