Hi, that is impossible, our plugin cannot delete itself and, anyway, the database is not touched during the update process. So all your data and subscribers is still theme, of course if you have not some kind of blog reset procedure which for sure is not part of WordPress or Newsletter.
Please, check your database, or ask a technician to check it for you, and look for “newsletter_*” tables. There you can find all your data. If those tables are not there you should definitively check what happened in your blog.
You can install Newsletter again, and it will show you the data store into your database.