This is for Ubuntu < 18.04. Ubuntu ≥ 18.04 no longer use Unity.
If your desktop seems to freeze for more than a few seconds, try
- press together the keys:
Ctrl Alt F1
- login with your username and password, and type
unity & disown
This leaves desktop/apps all as they were while fixing the broken state.
Restart the whole Unity desktop (which is like unto logging out and logging in, discarding your current desktop):
service lightdm restart