app_offline.htm is returned from the web server it is not part of Sitefinity project. I am not sure what could have caused this. Maybe the servers went broke and the hosting backued up the site to previous state since the current data was lost.
The hosting provider should have been making regular backups of the contents you uploaded (this is included in most hosting plans, but this could vary depending on the hosting plan). Can you contact the hosting provider and inquire about how can you obtain the latest backup.
The hosting panel may have an option that will allow you to download database backups. If not download the database and manually create a backup.
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