If you have completed the Sprint, then you must start a new sprint and add stories to it.
It is not possible to reopen a completed Sprint. Also when a story is done, within a completed sprint, it cannot be reopened too.
You can export a sprint to an excel file by going to the stories section and select the ‘more’ option above the stories. there you can choose to import/export
if you export all sprints to excel (even completed ones) and you want to upload all (or some) again to your sprintr then you just need to delete the id’s of the stories in the excel file. after you have done that, you can upload the stories, and since mendix doesn’t see an Id on them, it will import them as new stories
Since you have click “done”, and the sprint is over. you may have to start a “New” sprint instead.