Your current campaign has become corrupted and needs to be restored to an earlier state.
Before taking the below steps, please backup your campaign folder to another location.
Open your Fantasy Grounds data file. You can find your data files by clicking the folder icon from the Fantasy Grounds launcher window.
Locate your campaign within the campaign folder.
Save your db file somewhere safe outside of your campaign folder, then change one of the db.session files to db.
Relaunch Fantasy Grounds and your campaign.