I was on vacation for a week, turned the Mac off before leaving. Turned it on tonight and it said I needed to download "Security Update 1.01 and Safari update 2.0.1" or something like that. So it downloaded and then said that I must restart the computer, which I clicked OK. So it started to reboot and then it just stayed stuck on the apple logo screen with the little circle on the bottom with the moving lines. Then the fans started to run really hard so I turned it off. what can I do ????