The Tools menu Build Songs Database to rebuild where your files are located. If the External drive letter changed, then the files aren't where they were before. It may be easier to change the order you plug USB devices in to get the same drive letter for the ext. hard drive.
Read the Manual on Rebuilding the Songs Database.
Or, if you are using the Add Track section Audio, MP3G, Video buttons, just locate the directory you want to play a file from.