This morning, I started up my Mac Pro, and then went to take a shower. When I returned, instead of finding the login screen, I saw instead a grey screen with an icon in the middle. The Icon looked like it was shifting from an apple to a folder to maybe a question mark. I held the power button down to forcefully shut it down, then restarted. It did so normally, and appears to be functioning just fine. I don't know if it is relevant, but a couple of times a few weeks ago, I experienced the thing where your screen greys out, and you get this message instructing you to restart your computer immediately. Any idea what might could cause this? Is it possible that some component is in the middle of failing?