Whenever I tap the Super key to try and open the Dash/Search (I don't know the actual name), it just flashes once and disappears. I get the same results clicking the Launcher icon. I know it's not a problem with my keyboard because of that. Otherwise, it can be a problem with either the actual os, or a problem created by another program (possibly Unity Tweak Tool). And I think it sometimes works, too.
I had the same issue. The HUD used to work fine. I did install the Tweak tool (to get sloppy pointer focus).
Restarting unity (see How do I restart a Unity session from the terminal?) made the HUD come back.