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

1. press together the keys: Ctrl Alt F1
2. login with your username and password, and type

unity & disown

This leaves desktop/apps all as they were while fixing the broken state.

Full desktop reset

Restart the whole Unity desktop (which is like unto logging out and logging in, discarding your current desktop):

service lightdm restart