There have been a few time I can't shut down my computer. This means that when I hit Shutdown button, my computer does nothing. I can log off but can't shutdown or restart. If I want to shut it down, I have to hit my power button to turn off my computer which is not a proper way! This doesn't occur all the time but sometimes.
Please give me some advice of what I should do in this case: reading log, tracing, turning off process, etc...