1. Sometimes folders get moved around and Lightroom cannot find your original images. (It is very important that you do not rename any images until all editing is completed and you are ready to export your photos fro delivery). When LR cannot find the original raw images - a little tiny box will show up in the thumbnails. If you click on one of those tiny box icons - a message will appear asking if you would like to locate the original file? See image below..

2. Remember the image (file) number your are looking for and click on the "Locate" button. (Even if you have multiple files missing - only choose one). Navigate to to the folder where your original images are stored and click on the file number. You will be returned to Lightroom and all the files will update after being relinked to the originals.