Please answer the following so that I may track down the cause of this problem which I have not seen before:
So am I correct that all of the physical files are still present in each album folder and in the _source and _thumbs folders of each album?
Was auto approval enabled in the gallery configuration or had you been approving each file that was uploaded?
When you examine the _metadata.resources xml file(s) found in each album, did ONLY the approved date change or did other elements of metadata also change?
Do you recall having entered the configuration screen for the gallery and then clicked "Update" just prior to this happening?
Do you recall having done a manual file synchronization in the site's FileManager just prior to this happening?
Had you added any files to the gallery via FTP or any new albums via the user interface just prior to this happening?
Was a child album folder or individual files either moved or renamed outside of the user interface?