At times, when we attach a HUD, it may happen that it's out of our screen, and so it doesn't show. In these cases, something we can do is right click on the HUD in inventory, edit, and that gives us access to zoom out in the HUD area, the ability of repositioning the HUD, etc. (All viewers have the Current Outfit folder for direct access to what we're currently wearing. Then also, some third party viewers give us a Worn Items tab.)
But at times, it may happen that our HUD is within the HUD area... yet we cannot see it! What happened here?
Panic not. What happened, most likely, is having mistaken the shortcut to teleport back home, which is CTRL SHIFT H, with a newer shortcut, ALT SHIFT H, which allows to... show/hide the items on your HUD screen :-)
If you check your viewer's menu, notice that under the Me menu (official viewer; Avatar in other third party viewers), there's an option to the bottom that says... Show HUD Attachments (with the shortcut indicated next to it). If the option is ticked, the HUDs show in your screen. If it doesn't, the HUDs don't show, and that would explain why they don't show... even though you know they are there!
That's it for this mini-tip :o)
Other related posts on HUDs in this blog: Creator Resource: Creating HUDs - The builder's part