Please follow below tips to troubleshoot the issue.
1.Only site admin can start a restore for document library. Make sure you use site admin account to access the document library.
2.Go to Site contents -> Recycle bin -> Check whether deleted files are in the recycle bin.
3.If deleted files are not in the recycle bin, go to the Second-stage recycle bin, check whether deleted files are in the Second-stage recycle bin.
4.When you use the modern SharePoint experience, you can directly to the Restore this Library page by going to: https://tenant.sharepoint.com/sites/sitename/libraryname/Forms/AllItems.aspx?p=18
Note: Deleted files can’t be restored after they’ve been removed from the Second-stage recycle bin—either by manual delete or by emptying the recycle bin.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.