"Suddenly" is normally not the reason things stop working. What have you done before the problem started? Have you added new plugins or updated any plugins? It might help to disable other 3rd party plugins temporarily to find out if there is a conflict. Are the thumbnails only missing for existing entries in the files section or are they also not shown when uploading new image files? Do you see any entries in your server logs that might give more details about why the thumbnails won't show up?
check your error logs too
Solved it. Of course, it had to do with something I've done before, specifically a file in the engine/lib directory that became corrupt after some test editing.
Thanks for all the help.
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