I saw a similar report on Discord yesterday. It sounds like the campaign.xml file for that campaign got corrupted/erased.

Please ensure that you are not using a file synch solution on your FG data folder (like OneDrive or Google Drive), which can cause data corruption issues like this.
Then, copy the campaign.xml file from a new/different 5E campaign to restore.

You can use the folder button in the launch screen to access the FG data folder.