Hard to tell without knowing what was changed.
Can you show a screen shot of your relationship table. It's possible the relationship 'broke' somehow, probably by changing it.
Also, did you happen to change the name of the table the layout is based on?
Of course if you had a back up that works.... :)
Renaming a table or table occurrence (that's what we call a "box" in manage | database | relationships) will not break the connection between a layout and it's specified table occurrence. These links are managed by Filemaker through internal serial numbers so that name changes don't break the link.
Open Layout Setup for this layout and check the "show records from" drop down. What do you find there?
Did you delete any tables from manage | Database | Tables?
Did you delete any of the table occurrence "boxes" from Manage | Database | relationships.
Thank you PhilModJunk! I was able to select the table from the "Show records from" drop down. You saved me! I will definitely make a back up after fixing all the <missing tables>. Is there a quick way to fix them all or do I have to specify each field?
If you don't select the exact same table occurrence as was previously selected, it's a case of fixing one field and portal at a time.
You may find this method of generating regular back ups automatically during development useful: Saving Sequential Back Ups During Development