If your Outlook inbox suddenly looks different, the view may have been changed accidentally.
You can restore the current folder to its default layout.
Open Outlook and select the folder you want to fix, such as Inbox.
Go to:
View > Reset View
Confirm the reset when prompted.
Outlook will restore the default view for that folder.
If the Reading Pane Is Still Missing
Go to:
View > Reading Pane > Right
This restores the usual message preview on the right-hand side.
What This Change Does
Resetting the view does not delete or move any emails.
It only resets the way messages are displayed in the selected folder.