A couple of reasons this could happen, the files could be hidden, they could be in a format the viewer can not interpret for thumbnails (In this case you would see filenames but no thumbnails), Bridge maybe looking in an area of the drive your not(different folder and/or hidden folder)
Try a search in your file viewer (explorer or finder) odds are it will find them as long as they are not in a hidden folder. (it is possible to view image in folders that are hidden if you are in that folder. Which would explain why bridge can see them and your file viewer can not)