Photoshop Pro Selection Tool (Lasso) won't work?

I was using Photoshop Pro to edit a couple of photos where people blinked, and so I was using my lasso tool. All the sudden now, when I trace the line around the area I want to select, the shape appears while I'm tracing, but when I let go no dotted lines appear and it just goes back to nothing selected. What could be wrong--please help!