Hi. I had to do this. I think(!) I opened itunes, went to files drop down menu and clicked add files to library. You need to know where your music files are stored on your computer. eg mine were stored in my documents, my music. Sorry to be a bit vague - hope its some help
thanks evenstar that worked. however part of the reason i wanted to do it was to make more space on my hard disk , however even though i deleted about 300 duplicate songs i now have over 1Gb less space .aaaaaaaaarghhhhhhhhhhh!!