My iMac just crashed, desk top changes to a blank screen. Actually it has a triangle icon in the centre that when clicked starts Windows running in its partition, Windows is running perfectly. Turning off Windows gets back to the blank screen with the triangle. Then I have to cut the power to shut it right off. On booting it shown the Apple desk top for a short time then the Fusion partition seems to start by itself and get to the triangle icon screen. Any ideas how to fix it?