Note: this is for Ubuntu ≤ 17.04, since Ubuntu ≥ 17.10 no longer uses Unity.
If your desktop seems to freeze for more than a few seconds, trying pressing together the keys
Ctrl Alt F1
login with your username and password, and type
unity & disown
This will leave your desktop/apps all as they were while fixing the broken state.
To just restart the whole Unity desktop (which is like unto logging out and logging in, discarding your current desktop), instead type
service lightdm restart