Your key cylinder has broken, this is a somewhat common failure if you use the key lock a lot. It happened to me back in December, do a search and you will find my post about this. I got stuck at Ikea, had to call a locksmith, got a new key that week, stll haven't fixed the cylinder yet though.

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2002 X5 4.6is
1999 C5 Corvette
1992 Mitsubishi 3000GT VR-4
1994 Kawasaki ZX-9R
2005 Mazda 6 Wagon (Wife's)
1991 Mitsubishi 3000GT VR-4