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* The desktop environment session (KDE or Gnome) is completely frozen.
* Often, only the mouse is moving / responsive, but clicking anywhere doesn't change anything.
* It seems that only a hard reboot is the only option.
The linux kernel itself is very stable. That's why linux-based systems are used in professional settings (servers, super-computers, embedded systems, etc.) Thus, it is unlikely that the linux kernel itself is at fault.
Knowing that the linux kernel is still running is of little consolation when it seems that the only course of action out of a deep freeze is to do a hard reboot of the computer. Yet, as we'll see in the section below, the fact that linux is still running may allow us to recover nicely and, more importantly, figure out what cause the problem is the first place.
It is much more likely that a graphical application is consuming all of your computer's resources, leaving nothing to allow well-behaved processes to be responsive to your input.
More often than not, one process consumes 100% CPU, leaving nothing for the rest. We'll see below how to find the misbehaved process.
Sometimes, it may be an I/O bottleneck. This type of problem is usually temporary and usually indicates that the linux kernel is swapping memory to the hard disk. The solution is to prepare more RAM, or to close unused applications.
As we saw above, the linux kernel is very stable. Even when the system appears to be completely frozen, the kernel is probably still running.
We can use this to our advantage, provided that we have taken the appropriate measures beforehand. Often, it is possible to connect to a frozen machine from another machine in the LAN. If you own more than one computer which are connected together in your Local Area Network, make sure to learn how to connect via SSH from one computer to the other.
See sub-pages below for specific examples with given fixes.