You have just run a transfer, or a setup. You clicked the red dot and selected save.

When you rewore the HUD,

• you get the message to either start transfer or to setup, as if you hadn't already done so.
• when you press the toy button, it says it is not installed. It's gone.

Or, you re-wear the HUD after it detaches during Smart Reset. When you re-wear, some settings are lost.

Reason:

After the HUD detaches, you need to wait a while (half a minute's good) before re-attaching. When you detach any attachment (including your HUD), the sim you're in sends the object's data to the asset server. This is the "saving" process. Depending on sim load, this might be fast, or slow. If you re-attach your HUD before its data was successfully saved to the asset server, the asset server sends you the old HUD, that is, its state before the setup or transfer. Your saved settings are thus lost, and you are left with the HUD in its original, previous state.

Fix:

You need to setup (or transfer) again, as what you have set up (or transferred) has been lost by SL.

After you have done that, when you save, please take note of the warning in chat!

After the HUD detaches, wait 15-30 secs (or more if the sim is laggy) for the asset server to be updated, before you re-wear it.

If you're in a hurry, or you want to be sure, you can try this.