If you saved her from the demon she summoned when you first entered the house, I'm not sure what else could have killed her. There are not really any hostile creatures in Verdistis that could have wandered in. You may be able to bring her back using the procedure in;
Reviving dead NPCs

Did you install the patch before starting the game (some re-release versions of the game are already updated to version 1.0034A, shown in the bottom right corner of the main menu)? That adds protection for some essential NPCs so they can not be killed, but I don't know if Corinna is included, since she is not that important to the plot (though is a decent trader).