For viewing the document's history, open the document, go to 'View' Menu there you should see a menuItem 'History'. Clicking it should show you a panel containing various versions for your document.
For more info about this feature see this video.
adobe story team
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Adobe Story does maintain a history of your document. While in the editor go to View-->History to view your document's history. You can go back to a previous version by double clicking on its thumbnail in the panel that appears. You can also make it your current version by clicking on the Make Latest button on the top right corner.
In your case, when you went online your writing partner's version would have been saved as a version in the document's history. You can go back to it and get content from there or may be make it you current version by using the Make Latest option.
Let us know in case you face any issues while using this feature.
Adobe Story Team