I can't see while I'm working.

Maybe a couple months ago I opened photoshop and it looked normal but when I started working on a project the screen would go grey when I clicked on the screen to work. For example when im erasing a back ground and dragging the eraser across the image the work screen goes grey. When I let go the image or regular screen comes back up. So I can then see what I earased. But I'm basically working blindly and it is driving me crazy. Any idea on how to fix this?  P.S. I am no pro. I know the basics. I am working on a Mac.