When files get deleted from SharePoint, there is a limited time period when they can be recovered.
Detailed Information
Recovery by the Site owner
Initially, when a file gets deleted from SharePoint, it goes to the Recycle Bin for the site. A site owner is able to restore it from there if it has been less than 30 days from the time it was deleted.
Recovery by IT Admins
IT Admins have the ability to restore files from the Recycle bin if it has been less than 93 days since it was deleted.
WARNING: SharePoint is document storage solution, NOT a document retention solution. If you need long-term back-ups of files, then you should create a ticket with the Help Desk to discuss other options that might be available or would need to be procured, depending on your data retention requirements (time of required retention, size of files to be stored, the sensitivity of the files, etc)
Environment