The fix that worked for me right away was:
Start your machine and log in. Then change to tty1 by pressing Ctrl+Alt+F1 and log in on tty1. Then enter
sudo rm -rf ~/.config
Then restart your computer by entering sudo reboot.
Additional information: sudo rm -rf ~/.config deletes and thus resets your display settings and your audio settings, e.g. remove icons from your launch bar that are not default icons or unmute system sounds if you muted them beforehand. However, your installed software etc. remains untouched and unchanged. Check itsfoss.com for more information
The often recommended fixes did NOT work for me (source):
Suggested fix 1:
Change to tty1 by pressing Ctrl+Alt+F1 and log in.
export DISPLAY=:0
dconf reset -f /org/compiz/
setsid unity
Suggested fix 2:
Change to tty1 by pressing Ctrl+Alt+F1 and log in.
sudo apt-get install compizconfig-settings-manager
export DISPLAY=:0
ccsm &
Press Ctrl+Alt+F7 (or Ctrl+Alt+F8 sometimes) to get back to the graphical display where there should be a CompizConfig Settings Manager screen sitting there. Find the plugin 'Ubuntu Unity Plugin' and enable it.
I hope this makes is easier to find a possible fix for those who are experiencing the same issue as I had.
Best,
J