The thumbnails are still there but it says they do not exist. Just happened recently.
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If you really expect any decent answer, edit your question, at least to add OS and LO version, to better explain what the problem is. Otherwise, we’ll play guess game and many potential contributors have no time for that.
Thumnails ??? = destop icons?
X in the top of my documents
Then you succeeded in opening them and you say you can’t! Or is it an X across the icon? Then again what is the OS and your desktop manager (since you have the choice in some OS’es)?
… or are they the cross in the top right corner of the thumbnail in the start center, used to remove the entry for list of last documents?