Q: missing files in itunes

ive had all kinds of problems recently with itunes... i regularly keep my software up to date.  ive been using itunes to keep nearly 20,000 song organized since 2008.  now for some reason i cant figure out, many of the songs can not be found in itunes:  when i search for them in itunes, they are not there.  However when i search for them on the hard drive, they are exactly where they've always been, in the correct subfolder: User:Music:iTunes:iTunes Music:....

Another strange thing is there used to be a title at the top under all playlist called ALL MUSIC.  Thats gone now and that was how i searched my whole library.  Please help.  I've tried reverting back to previous versions of itunes but that didnt help.  Alot of missing file "!" are now in my library but are also found where they have always been originally.

thanks, brian