Best way to undo is to have a back up copy that predates the change. That can save you a lot of trouble. I use a system that saves a new copy with a unique name every 15 minutes precisely so I can "undo" any design change that I make. Here's how I do that: Saving Sequential Back Ups During Development
Here's what appears to have happened.
Go to your layout, enter layout mode and select Layout Setup...
Look in the Show Records From drop down.
This drop down may have the name of one of the Table Occurrence "boxes" found in Manage | Database | Relationships or it may show the same message. This drop down is where you select a table occurrence box from Manage | Database | Relationships to associate with this layout and provides the "connection" between your layout and the data in your tables. If you deleted a table occurrence box or changed the name selected in this "show records from" drop down, you'll get these kinds of error messages on your layout.
To fix it, you'd need to reverse the change you made. That may be as simple as reselecting the correct table occurrence in the drop down or you may have to add a new occurrence of that table in Manage | Database | Relationships and then select it the drop down.
Here's a tutorial on table occurrences that you may find helpful in better understanding what takes place "under the hood": Tutorial: What are Table Occurrences?
Thank you! I will take your advice next time and back up a copy before I work on major changes. Unfortunately, I didn't do that, but was able to use your solution to fix my table occurance. Certainly would have made my life easier if I had backed up first!