Thumbs not showing in WMC in W7

[jpcoh1]

I am a new user of this forum. Suddenly last week, all of the thumbs on my library of recorded TV programmes have disappeared when I view the list within WMC. All the recordings are on a separate drive. I did have a cloned backup of my SSD and I tried booting up from that. When I started WMC, it started regenerating the thumbs fairly quickly. However, when I reverted to my main SSD, once again the thumbs have disappeared. I am guessing that some of the files or other settings that make WMC work have become changed. Is it possible to copy across a whole set of WMC programme files from the backup SSD to the main SSD in the hope that this will restore the thumbs? If so, does anyone have a list of which files or folders should be copied? I have no real idea of what makes WMC work!
Thanks
Jonathan