My mom gave away my old dog, Andy, because there would be nobody to care for him while both parents were at work/we were at school. It seems stupid to me because we were at school for like 6 hours, we could just give him food in the morn then come back and give him some more after school. Maybe he was put down and she covered it up, I'm not sure. I think he was 6-7 though.
Sorry for your loss :(