Here's a mystery for you (and me). Suddenly, like today, I have lost an arrowhead and I don't know where it has gone.

I am using Photoshop 7 and today I started up PS and opened an image. I wanted to make a duplicate layer but the small arrowhead on the Layers Palette heading in the docking well has disappeared. In fact when I click on the Layers heading the docking well disappears instead of the drop-down palette. Can anyone please help?