Most likely, I will be able to restore the deleted data into the existing project. However, reintegration of data from backups is tricky. Make sure to download all data, so that nothing of the newer data is lost, should something go wrong. The questionnaire and its URL will be available after restoring the data.
In case that reintegration in the existing project should not work, you would have to merge the two downloaded datasets. I do not think that it will be possible, but ... better save than sorry.