My dog died and i dont know how?

I was eating and my dog was laying down, he got up and slipped somehow and landed on his side and laid and just started barking and crying for about 10 seconds and he stopped and laid there. i paniced because i didnt know what to do...and my parents told me he had passed away there. My dad checked his heart for a pulse with his stethoscope and told me he was gone. My dad took him away and i didnt know what he did with him, most likely to animal control to have him cremated. How could he have died? a stroke? a seizure? a heart attack? he also had a very large lump on his side for quite some time. but as he was laying there, he released his bowels so i guess that was a sign of sure death....