From MEPIS Documentation Wiki
What to do if your MEPIS system locks up. (If your system just became very slow, you may try killing a runaway program first.)
If you find your system has locked, chances are the kernel will still be fully functional, but the keyboard and mouse do not appear to work. This is usually due to a software fault in the GUI portion of your Linux system and in some rare cases a hardware conflict or a third party proprietary driver lock.
Unlike Windows systems, Linux has a proper emergency procedure to safely shut down and restart your system. In short, this is a keyboard sequence which gives commands directly to the kernel. It generally requires a little bit of hand gymnastics, but the benefit of this method is you usually do not lose any data or introduce errors to your hard disk through a hard reset or power down.
Press and hold both the CRTL + ALT + PRTSC (often labelled as SYSRQ) while tapping each of the following keys once only, pausing for a few seconds between each key press:
R S E I U B
The sequence is easier to remember with the "Raising Skinny Elephants Is Utterly Boring" mnemonic. Or: since the location of the S is not critical, some prefer to think of "busier" spelled backwards.
The final B will reboot your system.