@Alan Wright said
suddenly and without warning the format changed
Is there any chance that you had Classic Outlook and have been switched to New Outlook?
The header controls annoyingly no longer have the delete button which is now on a drop down menu.
I assume you mean that you are seeing this:
And you want to see this?
If so, you need to switch to the Classis Ribbon by clicking the little down arrow on the far right of the Ribbon.
comes up with Show on the right, Show on the bottom, Fill screen, pop out only or Resize. THERE IS NO "HIDE" OPTION
Fill Screen and Pop Out are about as close as you can get to "Hide". Try each and see which one is the lesser evil.