There are no tables on the relationship tab, only table occurrences. These are "labels" that refer to specific tables on your tables tab or, if using an external data source reference, in another table.
The good news is that deleting a table occurrence does not delete the table. Your data is still there. But each of your layouts refer to specific table occurrences as found when you open layout setup and check the "show records from" drop down.
To regain access to your tables, use the button at extreme bottom left of the relaitonships window to add a new occurrence and select the table from the list that pops up. Then either create a new layout that refers to this new table occurrence or update an existing layout to refer to it in Layout setup. If you choose the latter option, any fields on this layout will need to be double clicked while in layout mode to open specify fields so that you can select the new table occurrence in place of the old one that you deleted.
To learn more about table occurrences: Tutorial: What are Table Occurrences?
Thanks PhilModJunk! Not only was your answer correct, concise and complete, it was quick! May you have smooth sailings...