how do I retrieve lost documents
By definition of lost document they can’t be retrieved.
How do you define “lost documents”? Did you delete them, or have you simply forgotten where you put them?
Use the search function in your operating system or file manager. Remember to mount any network folders. In Windows you might have to widen the parameters used by Windows Indexing, see How to Search for text…. Sometimes a third party tool can be helpful.
Look in the rubbish bin, if you have one and restore from there
Look in backups or your LO Backup folder (folder defined in Tools > Options > LibreOffice > Paths) if you have set backup to be created in Tools > Options > Load/Save > General
Cheers, Al