Contact Microsoft Support for assistance with possible data loss issues. They will work with you to assess the best route to restore the desired content, whether that be versioning, the recycle bin, or an actual restore.Backups are retained for 14 days, so the sooner we can get started with data gathering the better.
By default Sites, Lists, Libraries, Folders, Items and Documents will all go into the recycle bin.
If an end user empties his recycle bin, the content will be transferred into the site collection recycle bin.
A site collection administrator is able to restore items from the site collection recycle bin.
Items that have been deleted into the recycle bin will remain there for 93 days.
The 93 days is the total length regardless weather the items are in the user recycle bin or in the site collection recycle bin.
Manage the Recycle Bin of a SharePoint Online site collection
It is possible to turn on versioning on documents and items in SharePoint Online.
If versioning is turned on and no limit has been set on the amount of versions to keep, you can go back in time for as long as you want.
If you delete something into the recycle bin, all versions will be delete as well.
If you restore something from the recycle bin back into a library, all versions will be restore as well.
If you have set to keep only 3 major and 3 minor versions, the oldest versions will be purged once a new version is being created.
Enable and configure versioning for a library
Versioning, content approval, and check-out planning
So let’s say that if a user deletes an item and waits 95 days, the item will be gone. If they contact technical support, we can request a restore from a backup taken up to 14 days prior, so we could restore the entire site collection back to a point where the item is still in the recycle bin and is capable of being retrieved. As we would be restoring the entire site collection, all data from the point of the backup to current would be lost.
Let’s say that we have turned on versioning and set to keep only 3 major and 3 minor versions. If we already created 3 major versions and we create a new one, the oldest major version will be purged. In order to restore this purged version, we would need to restore the complete site collection up to a point where the version is still there so we can restore it from the version history.
3rd party solutions
Note that there are 3rd party solution out there that are capable of migrating content from an on-premise environment into SharePoint Online. Usually if these tools are able to migrate content into SharePoint Online, they are able to grab content from SharePoint Online as well. Below are some 3rd party solutions that might be able to create interim backups of SharePoint Online as well.
There are also ways to manually export some of the data yourself using for example Outlook or SharePoint Workspace. Please see this post from a colleague of mine for more information.
Reference : https://blogs.technet.microsoft.com/akieft/2012/01/09/restore-options-in-sharepoint-online/