If you have a huge media library, especially if you download a lot of music, and especially if you use P2P like Limewire, chances are WMP is getting either bogged down with data or you downloaded a virus which is preventing it to open. Try accessing it from another user account on that computer. Create one if you have to. Then try opening WMP from there. If it still isn't working, try to re-install it by downloading from Microsoft website.
