Your file is probably 'kaput'. But you can try this
http://www.bruceshankle.com/_mgxroot/page_10765.html
Try opening the MSWMM file with this utility and re save it, and see if it comes to life. Oh, and please make a backup of the original file before opening it in this program.
You should always make a backup of your projects. If you can, stop using Windows Media Player, it sucks. I always recommend everyone that.
