what does a circle with slash mean on screen?

My MacMini server has been giving me problems. It won't stay on. First I got the folder with the question mark. Now, I'm getting a symbol of a circle with a line through it. I've been to the Genius Bar. They put the two disk drives as one, and put a clean copy of the system on it. I brought it home, updated the system as per the Apple store notification, put on Filemaker server, 1Password, and Dropbox. It's still not working. The tests they've run in the Apple store haven't come up with anything. But what does the circle/slash mean?

Thanks.