Try resetting your preferences. On the mac after you reset them restart your computer.
1. Exit the application
(on the mac restart the system)
2. Press and hold Shift+Ctrl+Alt (Windows) or Shift+Option+Command+Control (macOS)
as you start Illustrator. You are prompted to delete the current settings. The new
preferences files are created the next time you start the application.
If resetting prefs doesn't' work, please share some screen shots.
Drag the panel wider. Move over the left edge of the Pages panel. Look for the double-headed arrow, and drag left.
You can reset them by hand. Review PDF below. Can you drag the panel out so it is floating. You seem to be missing your scroll bar. Try resetting them by hand Indesign will ask you to reset with a dialog box so it did not reset.
Or try resetting your workspace. From the Workspace menu choose Reset [workdpace name]
Yep. first step prior to resetting the application.
I'm unable to find and delete the preferences file
Ok, so on Windows:
Delete InDesign Defaults in this folder:
Delete InDesign SavedData here:
If you can't find some of these folders, show hidden folders.
I was able to find and delete those files (thanks for the accurate and detailed instructions), and the thumbnails are now showing up properly.
Thanks to all of you for your help.
Great news, Issac. And we are always happy to help.
In this PDF I attached is the directions to reset by hand and my modifiers as well as the directions to reset every application in the Adobe family. This might be a good thing to review for the future.