If you don't see an item pane at all, you've simply closed it. You can reopen it from the View menu by going to View → Layout and enabling “Item Pane”.

If you do see the item pane but don't see any tabs, you've selected either an attachment or a note in the Zotero items list. Attachments and notes do not have full bibliographic metadata and cannot have other items attached to them, so you will not see the Info, Notes, Attachments, Tags, and Related tabs when you select them. Only regular items — with reference types such as books, journal articles, and newspaper articles — will show tabs in the right-hand column.

Tags and related items can still be associated with attachments and notes using the [Related] and [Tags] buttons at the bottom of the rightmost column.

See the library items overview for more information.