Model disappeared.  What happened?

This is weird.  I had this model open and created another few parts and came back to it to find that my dimension remained but everything else had disappeared.

I can make sketch items appear by hovering the mouse over the entry in the hierarchy.  All the Sketch and actions are grayed out in the hierarchy.  What did I do and how do I get it back?

Here is what it looks like to me.  Note the hierarchy on the left and the fact that all except the one I am hovering over is gone.

Magic???

Thanks for the help.

PS:  I am new to this, so it is not out of the question that I have done something, uh, stupid.