Try clicking on the “Add Business” button and then “Import Business”. Then navigate to the original location of your Application Data and import that business. If that is not the correct file, then try importing the file in your gDrive. Manager is good about not losing data, so you should be able to recover it.
That was for a previous version. The New and Open buttons have been removed so ignore any posts referring to them.
Other users have reported issues with using a cloud service and MacOS. I don’t know the outcome of that, perhaps other users will respond. See this post:
There is a guide to the Application Data folder here: Manage application data folder contents
There is also a troubleshooting guide that may help: https://www.manager.io/guides/troubleshooting/lost-businesses-or-recent-data-after-upgrading