I was finding that the left Alt key for opening the HUD was not working, so I went to Keyboard Shortcuts to re-set it. Selecting the shortcut to set it and pressing the Alt key just sets it to 'Disabled', however.
How can this be fixed?
Try using the dconf command, open your terminal to type:
dconf write /org/compiz/integrated/show-hud '["<Alt>"]'
This should restore the HUD default key to "Alt Left"