Blizzard have addressed issues players have been having with their UI profiles, where they would lose all of them. The problems are related to disconnects and there is a hotfix coming soon.
UI Profiles Missing Fix (Source)
the UI profile i made is gone and it reset to the default
We’ve seen this happen to players who experienced the bad luck of getting disconnected while loading into the game. We’ve developed a hotfix for that specific scenario, and the hotfix should be coming to all realms soon.
But the hotfix won’t retroactively recreate what was lost. To make sure you keep your custom UI (and so that you can use it elsewhere, if you like), you’ll want to export and save it outside the game.
While in-game, hit and then:
Select Edit Mode.
Select the layout you wish to export from the dropdown.
Open the dropdown and select the Share option.
Select the Copy To Clipboard sub-option.
Create a blank text file on your PC, and paste the string you just copied into the new file.
Save that text file somewhere safe.
To use a UI that you previously saved to a text file:
Copy the entire text from the text file.
Go into Edit Mode.
In the layout dropdown, select Import.
Paste the text from your clipboard into the Import Text window.
Give it a name and click the Import button.