When look the thumbnail, do you see any exclamation on the corner of the Images. if yes then you need relocate you images in Lightroom. Please Refer: Adobe Photoshop Lightroom Help | Locate missing photos
Now if you don't see any exclamation on the corner of the Images and you are able to edit your images in develop module only previews not generating under the Library, then try reset the Lightroom Preferences.
Navigate to the preferences file:
Mac OS: /Users/[user name]/Library/Preferences/com.adobe.Lightroom5.plist
Windows 7, 8: C:\Users\[user name]\AppData\Roaming\Adobe\Lightroom\Preferences\Lightroom 5 Preferences.agprefs
Drag the preferences file to the Recycle Bin (Windows) or Trash (Mac OS).
Let us know if it helps.
Thank you for your help. Unfortunately, that didn't fix the issue.
Please go to the below location and rename the folder "Lightroom 6 Catalog Previews.lrdata to Lightroom 6 Catalog Previews.lrdata.old".
Windows : C:\Users\Username\Pictures\Lightroom
Mac: MACHD<< Pictures folder<< Lightroom
Open Lightroom and see how it works.
THANK YOU SO MUCH! I'm not one to go around kissing people, but if I were, I'd kiss you! It worked!!
I had the same problem!! big THANKS