Very Unexpected Death

My kitty was 7 years old. He was not showing any signs of being unhealthy. Everything was completely normal the night before. My boyfriend and I came home from work Monday, and found our cat laying on a box, he was cold and stiff. He had probably been there all day, there was no food missing from his bowl, and nothing in the litter box. I just have no clue what could have happened. When we found him, I noticed his eyes were partly open and looked black instead of his beautiful green, and they looked bloodshot around the black. His mouth was partly open and I could see a lot of white in there with some blood, it looked more like vomit than foam though. There was also a spot on his nose that looked like something might have leaked out of it either while he was dying or right after he died. His tail was completely up and the fur on it was poofed out. Does anybody have any idea of what might have happened?

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