It appears you loose DWG file association with AutoCAD on your Mac.
You need to try this:
- in Finder locate one of DWG files, right-click on it and select "Get info"
- in Info window locate "Open with" section and select AutoCAD (which version you have) from drop down list (you may need to use "Other..." at the bottom of the list to locate AutoCAD on your Mac)
- then pick "Change all" button and confirm association.
Now test thumbnail appearance.
Maxim