An even better and cleaner method than the reset button is to use Ctrl+Alt+Del. When even that doesn’t work and the system has frozen, there’s always hope.
Hold down Alt+SysRq and press the following keys in order: R,E,I,S,U,B. Partitions will be unmounted cleanly, processes will be closed forcefully and the system should reboot soon afterwards.
of course, you need to “enable” this feature on the running kernel first !
On 2.6 kernel
echo 1 > /proc/sys/kernel/sysrq
will do the trick.
Some distros may have a way to enable this feature at boot time. On Fedora, edit the file
/etc/sysctl.conf, the line it sayskernel.sysrq = 0tokernel.sysrq = 1.