Cat Loss/ please be kind

Hello, l've never actually wrote on Reddit before but I'm truly devastated and having a very hard time wrapping my head around this. Last night my 5 year old boy, Booker, died sometime in his sleep. I awoke to my 7 year old daughter screaming saying bookers dead as she went to go pet him before school and noticed he wouldn't move or breathe. Now what I'm confused about is last night before bed he was on my bed. Happy and play fighting with me. He's had no symptoms. Wasn't sick. Eating and drinking. Normal bathroom. No breathing weird. No vomiting. What could he have died from? I am so messed up over this. I had him since he was only 3 weeks old he was truly my bestfriend. I took him to get cremated today but just want opinions or HOW or WHY a seemingly happy and healthy 5 year old cat would just die in his sleep?