Why did my cat suddenly die?

I am completely devastated. So I went to bed around 4 am the other night and I heard a commotion in the living room. I assumed this was my two cats chasing each other like usual. I got up to see what they were doing and saw my cat Domino frantically trying to open the cupboard under the bathroom sink. He got it open and jumped in there. I crouched down to see him and noticed that he was on his side and he started twitching violently. I was going to pick him up to rush him to the animal hospital but he wouldnt stop shaking so I didnt touch him for a minute. Then his head went back and his paw twitched a few more times then he stopped moving. I started crying hysterically hoping he was not dead but I just knew. Then I noticed his eyes were wide open and he was not blinking. I literally saw the life go out of him and I knew he was dead. I cant understand why this happened. He was only 8 years old and I loved him so much. I didnt even know if he was sick the only thing out of the ordinary was that he got kind of skinny. I didnt think anything of it because his weight would fluctuate so much. I just feel so terrible I dont know if theres anything I could have done to save him. Why would a cat just suddenly die?