WHAT
Microsoft Office (Outlook, Word, Excel, etc.) buttons at top missing. That area is called the 'ribbon'
WHY
HOW to switch back to old view
Outlook
- Click Home

- Click arrow/caret in top-right

- You are now using the 'Classic Ribbon' which MAY go away in the future
Method 2: click arrow box in top-right, 'Show Tabs and Commands'. Then click arrow in top-right.

then

FAQ
- Related KB's:
Zoom missing in Outlook-How to fix
Scheduling a Meeting in Teams - When I click buttons they disappear again! How do I make them stay put?
This indicates your Simplified Ribbon is set to collapse ('Show Tabs' only). Double-click Home OR right-click Home & turn off 'Collapse the Ribbon'

- The verbiage is confusing. Basically the ribbon (top buttons) can look 3 different ways:
<-- Simplified Ribbon, expanded (the default style/view)
<-- Simplified Ribbon, collapsed ('Show Tabs' only)
<-- Classic Ribbon (old standard that Microsoft may do away with)
- While ribbon is collapsed clicking Home will show buttons, but they disappear after clicking one until you turn off the 'collapse'/'Show Tabs' setting