Try to switch to a different workspace Ctrl+Alt+one of you arrow keys, unless your system is completely frozen it should be able to switch workspaces
In the new workspace look for System Monitor in the dash, it is like Windows Taskmanager.
you should then be able to "kill" the process by right clicking on it.
If this doesn't work you can restart your desktop without having to do a hard shut-down by logging into one of your virtual desktops (Ctrl+Atl +F2(or 3-6)) then enter the following commands
sudo stop lightdm
you will get a notification: lightdm stop/wait then enter the following
sudo start lightdm
this will restart your Xserver and send you back to the login screen on your desktop or straight to your desktop if you log in automatically.
Note; this is still a semi-nuclear option, it will kill any programs that are running and you will lose any unsaved work but it is safer for your computer than holding the power button.
There is also a text based task manager called "top" that you can access from a terminal or your virtual desktops but it's directions are rather long, you can get information on how it works by entering this into a terminal
and the command to launch the actual "text task manager" is: