Smart High-Side Switch - Reverse Battery Protection Feature
On page 8 of the IC's datasheet it is stated that in case of reverse battery, the load current has to be limited by the load. 



This implies that an additional current limiting mechanism must be implemented at the load side, in order to limit the back feeding current, which is confirmed on page 5: 



Most likely (according to your description), since the temperature protection is not active in this (reverse battery) operating mode, the IC could have run into a thermal runaway situation, leading to its self destruction.

If at the moment of your test, there was a non-limited low-ohmic load connected to the output, it could have fried the internal circuitry. As for the current behavior (zero output voltage), you would have to know exaclty got damaged inside the IC.