What should I do when Ubuntu freezes?
System freezes and even seconds in clock stop. Keyboard no responds only mouse pointer moves but no left nor right click. How to fix?
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
Does this happen always after an specif event, or out of the blue?
Have you tried restarting the window manager lightdm from a tty?
This will close your apps though.