Sometimes you may experience the screen ‘freezing’ and you are unable to click on anything.
The first thing to try is holding down the Alt key and tapping the Tab key to cycle through the open programs.
Often what happens is a program has opened/a dialogue box has appeared, but it has not come to the front of the screen.
When you complete any actions that that program/dialogue box requires, you will then be able to use the other programs again.
If this fails to resolve the problem, if you press Ctrl, Alt and End at the same time, it will give you the option to log off. This will close your session and any programs that you have open.