Welcome to the club... I have a 1991 C4 Cabriolet that suffered the same problem. One day I was driving normally to work and when I got out to head back home I could not shift it.... Lucky for me, it got stock in second and I was able to drive it home. I thought it was the bushings on the shift linkage but not the problem... Car is parked until I have time to drop engine and tranny to check in more detail. Let us know what you find.
Here is my post when it happened:
Shift linkage bushing - Rennlist Discussion Forums