Do you mean that in LR you see "?" on the folder names? If so, it's because LR can no longer find the folder using the same drive name (or letter if Windows) and folder path. If you replaced your crashed hard drive and re-populated it from a backup this is typical. Right click (or Ctrl+click on a Mac) on the highest folder that has a "?". This will pop up a Finder or File Manager window. Navigate to the folder on the new drive and select it. The "?" should go away. Repeat with other folders as needed. If all your image folders are under one master parent, you'll just need to do this on that one master parent.