Occasionally the mouse cursor will freeze - I have to reboot.
In Ubuntu is there a keystroke combination such as Ctrl+Alt+Del which would enable me to continue working without having to reboot?
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The first thing to try is to press Ctrl+Alt+F1. If the GUI is frozen but the keyboard driver is still responding, this will switch to a text console. You can log in to that text console and access your system and try to solve the problem. You can return to the GUI screen with Ctrl+Alt+F7 (some distributions may use a different number).
If that doesn't work, your computer may still be running, but with the console completely hosed. Then you can still access it over the network, provided you have an SSH server running. Log in from another computer (PC/tablet/phone/…) and try to solve the problem.
If you have no way to access your computer because the console is unavailable and you didn't have an SSH server, you can try killing the whole GUI session with the magic SysRq key Alt+SysRq+K: press and hold Alt, press and release SysRq, press and hold K, and finally release Alt. Depending on your distribution, this may or may not show a new GUI login prompt after a few seconds. If you don't, use Ctrl+Alt+F1 to switch to a text console, and Alt+SysRq+K to switch the keyboard to the text console mode. On a default Ubuntu installation, I think the command to restart the GUI login prompt is
service gdm restart.
If that doesn't work and you just want to reboot, you can use the magic sysrq keys E, U, B:
(Do not use or try to remember the popular “REISUB” sequence. The R and S do nothing in this sequence, and the I defeats the purpose of the previous E.)
As to what is causing the GUI freeze, the most common culprit is a buggy 3D video driver. 3D video drivers are very complex and video card makers don't provide good support, so the drivers tend to be buggy. Often, the 3D effects are provided by the Compiz program, and killing the
compiz process unfreezes the interface. To kill the Compiz process, once you have a command line access, run
If you find that you often have freezes due to Compiz, switch to a different desktop environment that doesn't use Compiz. The Ubuntu default (Unity) requires Compiz, but most don't. Try KDE or LXDE (without enabling Compiz of course).
If you can still reach a console or terminal, maybe you could try this solution. Otherwise I believe sebelk's solution is probably a good start. If killing the X server is not sufficient and you can't reboot properly the machine, then in the same idea as sebelk, you could at least try to raise elephants to reboot more properly.