Never mind!! I just shut down LR and restarted it and the folders came back.
So you did not move any images across folders, they just do no longer show up in the left-hand panel "Folder" in Library module?
Click on the "empty line", so that the triangle no longer points to the left, but down. Does this restore the folder structure?
If not, right-click on an entry underneath and choose from the context-menu "Show Parent Folder" or "Hide this Parent", until the displayed hierarchy suits yourself.
Thanks for the response. It was kind of odd. My cat walked across my computer and must have clicked the "minus" sign on the folder section. It removed everything in there including the hard drive leaving the section blank. My photos were fine, all of them there there. Even with the triangle down, it was just blank. I was not able to add any folders or subfolders or connect back to my hard drive. It wouldn't let me import the folders again although it found them in the browser. You could see the folder name along the bottom above the filmstrip and it would jump to the folder in the library but no display in the left pane. I searched a lot of forums but didn't see anything similar. Finally I just exited Lightroom and relaunched that catalog and it restored the link and the folder structure. I guess the easiest fix is just log out!