If you've been using the disc for a while and it suddenly stopped working, it could be that it's just aged too much and becoming unreadable. It happened to mine, sometimes you'll see the edges start to discolor or even peel. Being subjected to extreme temperatures in a car will take its toll on CD/DVD media.
On the other hand, if you just burned it and it wont work you either have a bad CD image or your brand of media isn't supported by the BMW reader. It's strange, but it is sensitive to brands. I noticed when I burned mine that it does NOT like Memorex (other members have also indicated as such), so stay away. Verbatim works great for me.
Either way, definitely burn another copy.