Could it be that you are in the full code editor view? If so, go to View > Code editor and switch the toggle off to return to the view with the GUI styling options.
Or have you clicked the Microthemer logo at the top left of the interface? If so, click it again to un-hide the interface.
If it’s not either of the above, could you send me access details for the affected site via our secure contact form so I can see what’s going on?