jsFiddle has recently (actually quite a while back) switched to a new design, and the old design (cached by the browsers) stopped working (CSS, HTML and javascript all showed a blank frame).
If you see a blue favicon (the old one; the new one is black) and a toolbar with gradient (the new one are just single-color rectangles for buttons), it's time to reload. Not an ordinary reload, you need CTRL+F5 or CTRL+SHIFT+R
