What can you do when linux is not responding

Linux is a very stable system compare to windows(:P). However even it is very stable, it will still crash sometimes.
So what can you do if the system is crashed.

1. go to the tty:
Sometimes, it is the graphic mode crash but the linux kernel and other critical process are still alive.
If you have encounter this situations, There is no need to reboot your system.
Instead, you can goto tty to fix the problem.
In linux system you can go to tty by pressing [ctrl]+[alt]+[f1~f7].
The default graphic mode is in tty7, that is you can press [ctrl]+[alt]+[f7] to return to graphic mode.
A tty is a pure command prompt and you can restart the x-server from here.
ubuntu 11.10 use lightdm, so I take lightdm as an example.
type the following command:
sudo /etc/init.d/lightdm restart
that's it, and you will see the graphic mode is restarted.

2. the magical sysrq:
If your system crash and the keyboard has no respond, it's time to use the sysrq.
What is a sysrq, it is a little button on your keyboard. Normally, it is near the delete key .  If your system crash, and you can not enter the tty mode. Try the following combination keys:
[alt]+[sysrq]+[R]-> [alt]+[sysrq]+[E]-> [alt]+[sysrq]+[I]-> [alt]+[sysrq]+[S]-> [alt]+[sysrq]+[U]-> [alt]+[sysrq]+[B]
If everything works fine, your system will reboot but will save some files and safely kill the process that you are working on.
So what the hell is going on under these combination keys?
The following show you the functionality of each keys.

a. [alt]+[sysrq]+[R] : turn your keyboard into ascii mode, it enables your keyboard to send message to the kernel directly.
b. [alt]+[sysrq]+[E] : send SIGTERM signal to all the process except the init process.
c. [alt]+[sysrq]+[I] : send SIGKILL signal to all the process except the init process. This will kill all the processes except the init process.
d. [alt]+[sysrq]+[S] : sync the buffer pool to the hard disk, in case to lose datas.
e. [alt]+[sysrq]+[U] : remount all the mounted-filesystem to read-only.
f. [alt]+[sysrq]+[B] : reboot the system.
p.s while using the combinations, use it slowly. :P
That is, after using the first combination, wait about 5 secs and then use the second one and so on. If you use the combinations too quick it is no difference than press the power key. 
The recommend wait time is:
R--1 sec-- > E--30 sec --> I-- 10 sec --> S --5 sec --> U -- 5 sec --> B

If you want to know more detailed about the sysrq the following link has a very good explanations.
English version:
Magic sysrq
Chinese version:
https://www.deleak.com/blog/2010/10/20/sysrq/