All Records Suddenly Disappeared

I have Ninox Database Premium Version 2.6.7, and I am running it on macOS 10.15.3 (Catalina). Today I was editing a database that I have been working on for years. I clicked on something, and all the records suddenly disappeared. I tried clicking “undo,” but that did not help. All my other databases still have all their records and work fine.

The last thing I remember doing, was trying to edit a field to change it from Yes/No to a checkbox. I clicked the gear icon, selected ‘Edit fields’, and then all the records suddenly disappeared. Now there is a filter icon next to the field name that I was editing, which was not there before.

Any idea what might have caused this, and how I can get my records back?