Unexpected loss, so confused😔

I adopted my sweet 1.5yo tabby just 5 months ago and he was already super bonded with me and my older cat. This morning he didn’t come running for breakfast, so I went to look and found him lying in the floor in the next room and realized he had died. Last night he was his obnoxious zoomy lil self so it was entirely out of the blue. I’m just so stunned and sad and utterly lost as to what happened