I have a new iMac 27'' late 2013 which I purchased almost 2 months ago. Today I experienced a problem I haven't come across before. While the iMac was idle with programs running and sound it tried to get to sleep or screen saver mode and the screen went black with the 4 four edges of the computer white. I wasn't able to bring it back by using the keyboard and the mouse so I pressed the power button and restarted. My keyboard and mouse are both Logitech just to inform. I have Mavericks fully updated. How can I fix this issue and what might be causing it?