Cause: Another user may have stopped sharing the workbook
Solution: Regain access to the shared workbook
Someone with full read and write access to the shared workbook has changed your permissions, so that you no longer have write access to the file. If sharing was stopped while you were editing, you'll see the message even after the workbook is shared again, and changes from your current editing session will have to be re-entered.
The only way to regain access to the shared workbook is to regain write permission for that file in the Mac Finder. Contact the other users of your shared file, and request to have your write permission restored. Close the workbook and follow up with the owner or other users.
Cause: You may have been disconnected from the network
Solution: Connect to the network and then reconnect to the shared workbook.
Your connection to the network share may have become broken, disconnecting you from the workbook, or another user may have disconnected you. If the workbook is still shared and available, you have two choices:
You can open the shared workbook again, but any unsaved changes you made in your current editing session will be lost.
You can save your unsaved work, and then merge it back into the shared workbook.