Shifter cable broke down: misuse, lack of maintenance, shifter or regular occurrence?
It's not uncommon. Some Shimano shifters are prone to breaking off near the head in the shifter and it can be a bugger to get that little bit out of the shifter. My wife has Ultegra 9sp and I have to replace her cable about every 12-18 months.

She can now tell when it's beginning to fray because the shifting gets dodgy before it completely breaks.