Linux machine hangs.

hey guys What all things one should check if a Linux system hangs? After rebooting, there were some bad block messages, but other than that the system was pretty much ok ....... but generally what all things should one check from a general and security point of view when a system hangs? Ofcourse "security" is an important part of the "general" things :) thanx in advance, Abhijit. -- ~~~~~~~~~~~~ expect nothing and be surprised ~~~~~~~~~~~~~