A tough one ... it happened in my sector not that long ago .... the funny thing is ( and correct me if I'm wrong) that at least EU requirements(or recommendation) for the upper airspace in regard to radar equipment/coverage is : 2 SSRs are legaly sufficient for the provision of ATC services (PSR is required only in the lower airspace ) .
So guys - hows the situation in that regard at your units?
The fact is - transponders do fail from time to time no matter how unbeliveable that is.
1999