This error means that your database file has been corrupted. The solution is to restore a backup. Under the "File" menu, click on "Restore" to see a list of recent auto-backups. Select the most recent backup and click the "Restore" button. If that doesn't work, try the next most recent auto-backup, etc.
Speaking of backups, make sure that you're keeping your TntConnect database on Dropbox. Dropbox is a cloud-based service that can safely and securely backup your TntConnect database to the cloud so that your database is safe in case your laptop is lost, stolen or damaged.
You can sign up for Dropbox for free here (and get 2GB of free storage). Dropbox works with both Macs and PCs. Dropbox can also help you synchronize your database between computers.
Once you have Dropbox installed, under the "File" menu, click on "Resolve Dropbox Conflicts" and TntConnect will help you move your database to your Dropbox folder.