Frozen Screen Mac book air

If you are unable to do anything, type or move your mouse cursor, it hanged. Maybe due to lack of disk space, high memory usage or high CPU usage. Try to restart your Macbook by holding down the power button. And see if you still have a frozen screen. If you still have a frozen screen after the restart, contact an IT if you still want to use it or buy a new one.

If you the frozen screen is gone or if it is working properly, download a cleaner application from the App Store to clean your mac. Just type "Cleaner" and App Store will give you a ton of choices.