MC should read all of the files from the directories that you do import on. Do you see files on disk that are not in MC's inventory? Are they in the directory or directories that you ran import on?
It's possible that you have some files that have missing or incomplete metadata. Find a few examples of albums that you can find on disk, but you don't see in MC. Then, in MC's search box, type part of the file name. See if the file or files show up then. If they do, then it's a metadata problem. If MC doesn't have them at all, then you probably need to import the parent directory where these files are located.
It's hard to say what's going on without more information.
Did you have a disk crash? Or move around files? Or move or lose an external disk? Or rename a disk? I'm wondering why your library became completely inaccessible in the first place. That seems like the right place to start.
Brian.