I have an 01' Volvo. It has an alarm siren under the bumper. They put a nicad battery in the siren so that even if the main battery is cut the alarm can go off. This battery is prone to going bad, leaking out, and ruining the control board. This will cause the sunroof to not work. The rest of the car is still fine. You will get an error message about the security system, but the car will start. You just can't use the sunroof.

If you catch the problem in time, you can cut the siren open and replace the battery, there is also some way to eliminate the alarm feature through reprogramming I think. I have also seen an aftermarket board that will take it's place.

I just live without the sunroof as it's not worth fixing.