For situations like...
Your mouse pointer moves but you can't interact with any windows.
Press ALT + F2. In the Enter a Command box that should appear type r and press Enter
If that doesn't work (e.g. it sometimes seems to fail if you are running Gnome over VNC)...
killall -HUP gnome-shell
Later versions of Ubuntu use "Wayland". In this environment restarting using Alt+F2 and r is not possible.
However using Super (aka the Windows Key) +L will log you out without stopping running programs, which should have a similar effect
gnome-shell --version
apt list --installed gnome-tweak-tool
sudo apt install gnome-tweak-tool
gnome-tweaks
apt list --installed gnome-shell-extensions
sudo apt install gnome-shell-extensions
Restart Gnome as described above. New Extensions should now appear in Tweak tool...
Search or browse for the extension you want
Download. Enter your Shell version if required. (gnome-shell --version)
Extract the downloaded file to a temporary folder
Check the UUID from metadata.json in your temporary folder (grep uuid metadata.json)
Move your temporary folder to ~/.local/share/gnome-shell/extensions and rename it to match the UUID
Restart Gnome Shell (Alt+F2,r)
Restart Gnome Tweaks tool. The new extension should now appear.
sudo apt-get install chrome-gnome-shell
Use gnome-tweak-tool or...
dconf write /org/gnome/desktop/interface/clock-show-date 'true'
tracker-miner-fs
tracker-store
tracker-extract