check the folder view first. From an explorer in the Organise menu select folder and search options go to the View tab, in there see if the first thing [ ] Always Show Icons not thumbnails is unchecked . . If it is already unchecked , switch it one way, and ok your way out, then switch it back. . . In the same place temporarily turn off [ ] Display file icon on thumbnails which is an overlay, that shouldnt effect it.
Per User setting for the same thing. In the start menu find computer right click bring up the properties in there click on advanced system settings go to the advanced tab, in performance click on settings in the Visual Effects tab you have [X] show thumbnails instead of Icons check it. Now this makes no sence, because I had that unchecked, and I am seeing the thumbnails?
Cleanup Thumbs: Next is to try and clear out the old thumbnails. find the diskclean tool by typing this into either search or run
cleanmgr.exe , select the drive your having problems with, then clean up the Thumbnails.