The newest iteration of the official SL viewer has an alpha sorting bug which is causing HUDs to lose their buttons, among other things, like so:

If this looks awfully familiar, here's how to fix it.

If you don't already have it enabled, hit Control-Alt-D and enable the Advanced Menu:

Now, from the Advanced Menu, go to Rendering and check "Fast Alpha" (No, it's not checked in the image below.)

Tada!  HUD fixed. :)