Thumbnails Randomly Updated?

Unfortunately a couple of years back we were the victim of Ransomware attack on our NAS drive.  We were not completely backed up and lost a year's worth of photos, some of which I was able to recover from other sources.   For those that we could not recover, when I went into EO 2018 the thumbnails were all still intact/displayed even though the actual photo was no longer connecting.  I was able to go in, click on the thumbnail and do a screen shot to recover some photos even though the low resolution was fuzzy but better than nothing.  

Life got busy and yesterday I  finally decided to go in and capture some additional photos only to find the thumbnails for those missing photos no longer are being displayed.  The thumnails are now ? but still pointing to the missing files. Not sure what happened or if the cache got recreated somehow unbeknownst to me.  I hadn't backed up the catalog unfortunately.  Any ideas what happened or if there is any chance of recovery?  Are there other events that cause the thumbnail cache to be regenerated?