What should I do when Ubuntu freezes?
I use ubuntu 11.04 64bits.
Sometimes ubuntu will get stuck that everything paused including mouse,keyboard...etc.
How to know what happened?
How to fix it?
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.
By "getting stuck" I assume you mean "getting stuck temporarily", the system freezes for a moment, and then the reactivity is back. In my experience, such behavior is caused by extreme high system load, especially IO operations (writing / reading from disk) due to high memory usage and the necessity of using swap.
If this is a thing that happens on a regular basis, I would start monitoring the system load and what programs consume the CPU and memory.
Start by installing
Alternatively, use gnome-system-monitor.