The HUD is gone / mispositioned?

So I think I've broken the game a little. The entire HUD seems to have vanished, and there's no option setting, or button combination to get it back.
When changing zones, a little of the HUD comes back, but is in the completely wrong position:

So my questions are:

• What causes this?
• How can I fix it?

Solution 1:

As of latest patch, this should now be fixed:

Fixed an issue where the HUD would get permanently disabled for some players.

Source

(See edits for the cause of this bug)

Solution 2:

I've had this issue before, it looks like my HUD was placed as if my screen was using a lower resolution. Quicksaving and reloading did the trick for me. As for what caused it, I'm not sure but it may have to do with me alt-tabbing quite a bit (I have a dual screen monitor and I hate not being able to play this game in windowed mode so I can use my 2nd screen).