In the Library module, right-click on the folder with the question mark and select "Find Missing Folder". In the window that opens navigate to the folder and select it. If LR lost contact with the folder because you moved it, it will now know the new location. In the future you can avoid having to do this by only moving things, folders or photos, inside LR, not outside it.
If the problem was that the folder is on an external drive that was not on-line, while the drive is connected you can make Smart Previews that will be used when the original files are unavailable (and in the future build them during importation) and when the full sized files are once again on-line, the editing done on the SPs will be associated with the originals.