Although very rare Linux might get stuck as well and won't respond to keystrokes or other user input. In the worst case there is a kernel crash, sometimes with a kernel crash dump on the console, probably caused by an unstable driver or hardware problems.
If you are lucky the system stucks but you are able to login from a remote machine by ssh. You can kill the process
kill -9 ... or reboot the system
shutdown -r now. Most of the times a jam is caused by one or more processes consuming too many resources. Symptoms are extreme high CPU usage, high load, high memory usage, a high queue of waiting processes etc. If there are no resources left the kernel might still be running but the system doesn't respond to keystrokes,
CTRL+ALT+DEL won't help and login from an other system with ssh is also impossible, as there are no resources left to start a new shell.