Thumbnails.....
Are the files themselves still intact ?
It could just be the file generated by the operating system became corrupted which is no biggie to fix.
When in explorer browsing the folder change the folder from thumbnail/preview view to that of details - then try accessing each individual file - if you can open each individual file then these are intact and its a corruption of the thumbnail preview performed by explorer.
If its a corruption of the preview open the folder in explorer, navigate to the folder of pictures and then try hitting ctrl-f5 - this should force a rescan of all files and pictures.
If this does not work you can try deleting the preview file that the operating system builds to speed up folder scaning. Whilst in the folder of picture change the view from preview/icons/thumbnails to detail and ten go to the tools menu, select folder options and then go to the tab view - uncheck hide extensions for known file types and change the option to show hidden and system files - then look in the folder for thumbnails.dat - delete this file (it will be slightly opaque to signify a system file). You can now switch back to your preview/icons/thumbnails view and hit ctrl-f5 and the file will be rebuilt - the pictures should no longer be corrupt.
You may want to reverse out the changes to folder view by reversing what I said above in the folder options so all looks familiar again.
Hope this helps.
ps. Thumbnails.dat is one of the very few system files you can get away with (and windows will allow) deleting as it is generated from your content in the folder.