Ok, this is my big big problem.. One day, i was just doing nothing on my dear computer, starting up firefox or something, then suddenly everything stopped. I couldn't do anything. I forced-restarted the computer with the boot-button. When im trying to start it over again, i get to the white startup-screen with the apple-icon in the middle, with the loading-circle underneath it. After a while, the apple-icon changes to a grey stop sign (picture of it further down). So,...