What do I do if Ubuntu 16.04 freezes, but I can move only the mouse?
Well there is a magic key combination (to reboot):
While holding Alt and the SysReq (Print Screen) keys, type REISUB
For more details visit this answer : https://askubuntu.com/a/36717/497359
Switch to the console: CTRL + ALT + F1
After login, you can see which processes are running using this command:
If you see a process which has high CPU usage, you can kill it.
sudo kill <pid>
Of course, you can simply restart the PC too: