First, confirm that the folder is where LR expects it to be by going to the Library module, right-click on the photo, and select "show location in Explorer/Finder".
The next step probably involves finding "missing" files that have been moved or deleted from outside of LR.
Now it displays the photo in the small window but not in the main window...
Sounds like it might be a video card communication problem.
Go to the Preferences dialog, Performance tab, uncheck the GPU usage.
Restart LR and see what happens.
Thank you so much!