Try opening with the option key pressed. You should get a dialog where you can choose which catalog to open. Choose yur catalog and select "test integrity of this catalog". It's a long shot but it will try and repair damage to your catalog file with this option.
Two very important things to check if you are on Catalina is that the app's access is set correctly in System Preferences -> Security & Privacy -> Privacy -> Files and Folders as below:
Lastly, if you are using external disks that are NTFS formatted, you might run into the problem where the diurvers you were using for that need to be updated. Mac OS X cannot natively write to such disks and those disks should be reformatted to HFS extended or APFS when you get them first or you need to use a third-party driver.