How to Fix Missing Game HUD IssuesEdit
If your game HUD does not attach automatically, try going back to the Linden Realms Portal Park. Try a different portal and see if it attaches.
Each portal leads to a different 'instance' or copy of the island. In the landing area of each island, there is an object responsible for attaching your HUD. If that object is malfunctioning in one instance of the game, using another portal and going to a different instance will usually fix the issue.
How to Fix Stuck Game HUDEdit
If your game hud stays attached even after you leave the Linden Realms region, just wait and it should detatch itself automatically. When it does, there is currently a bug that will launch you up in the air, but it's not a big deal.
If you've tried and tried to detatch your HUD and it just won't go away, you can usually get it to disappear by logging out and then logging back in again.
Going on Without a HUDEdit
It appears you can still collect crystals without a HUD. You need the HUD to finish quest objectives, but not to simply collect crystals to exchange for Linden Dollars. You should test this out before you spend hours collecting as the situation may change, but for now it works.
Without a HUD you won't know how many crystals you have, but if you stand on the Crystal Collector pad you will still exchange crystals for Linden Dollars, provided you have enough.