Why did my rat die all of a sudden?

I woke up this morning.. and Otis was fine. I went to shower and when i got out i heard a "thump" coming from my office where he lived, I payed no attention really and was talking to my wife on the phone when i looked ove r at his cage, and i noticed a blood puddle slightly larger than a nickel at the top floor of his 4 floor cage. i looked down and he was laying on his side on the bottom floor of the cage, eyes open, not breathing... he had no signs of lesions or abrasions, no growths or tumors from what i could see. he had been a little less sociable the past 2 or 3 days, sleeping a lot and not really movin around. when i picked up his food bag this morning he just looked at me, when usually he goes running up the floors to his food bowl. can someone tell me why my best lil buddy died all of a sudden? this is devastating to me.. he was only 1 1/2 years old, i fed him properly with minimal breaks of the diet for treats, i played with him a lot, everything.. i dished out 150 bucks for his cage.. after he outgrew 2 other ones, i bought him a female companion about 2 months ago.. that he never mated with, and she seems to be fine.. why did he die?