I'm on my second Mac Mini over three or four years. I've just started getting hard crashes by which I mean the screen goes blank, you get a message about a problem, and it restarts. This is on Yosemite. Once it crashed during a World of Warcraft session and another while doing a Sudoku puzzle in the New York Times. Wow is resource intensive and the puzzle uses Java. They don't seemed related. Finally, I found it at the log on screen in the morning after not using it since the night before. Does this sound like a hardware problem? The mini is about two years old.