When I open Libre thumbnail pictures of files show. When I click on one to open it , it says “FILE DOES NOT EXIST”. How do I open ?
What you call the “LO thumbnail picutes” is likely what is called the Start Center. The Start Center simply displays a list of recently opened files. It is strictly equivalent to a text list, without any commitment with the OS file system.
In other words, if you move the files, delete them, rename them in your OS file browser, or more generally outside LO, LO is not aware of these modifications and does not update the list to match the OS file system.
An alternate way to open a file is to navigate to its directory with the OS file browser and to double-click its icon.
PS: when asking here, always mention OS name, LO version and file format (when question is related to a specific file).