Moodle 2.6.2
Clean Theme
GradeMe plugin had just been installed. It was working in 8 other courses. We added it to a course with 60 students and it was displaying fine. Then dragging the block from the left side to the right, caused the whole course to go white blank screen.

I unistalled the GradeMe plugin and then the course was back to normal.
Someone else had a similar post in the forums in November 2014. That's how I knew to unistall.