There can be several reasons why folders are not showing in Outlook. Here are some of the major causes:

1. Folder view settings: If the folder view settings are set to a different view, such as "Unread Messages," "Flagged Items," or "By Category," it can hide folders. To check this, click on the "View" tab in Outlook and select "Change View" to see if any other view is selected.

2. Navigation Pane settings: The Navigation Pane might be set to hide folders. To check this, right-click on the Navigation Pane and select "Options." Ensure that the "Folder Pane" is selected and the "Show Favorites" option is checked.

3. Corruption in PST file: A damaged PST file can prevent Outlook from displaying folders. To check for this, use the ScanPST.exe utility to scan and repair the PST file.

4. Hidden folders: Outlook can also hide folders that are marked as hidden. To view hidden folders, click on the "View" tab and select "Folder Pane" and then "Options." From the options, select "Show all folders."

5. Account configuration: If the account settings in Outlook are incorrect, it can prevent folders from syncing correctly. Ensure that the account settings are configured correctly.

6. Conflict with third-party add-ins: Third-party add-ins can also conflict with Outlook and prevent it from displaying folders. Try disabling any recently installed add-ins to see if this resolves the issue.

These are some of the major causes for Outlook folders not showing. If the issue persists, it might be necessary to seek further assistance from Microsoft support or an IT professional.