This has nothing to do with what happened. I had this same problem mere moments ago and can't seem to find the problem.
I installed a new widget, clicked on the dashboard icon, and everything was there. I clicked on the widget I downloaded (Screenshot Plus, I have used it before with no problems) and then all of my widgets disappeared. The dashboard screen is just the grey dotted background with only the right facing arrow in the bottom right hand corner. There is no plus sign in the bottom left or anything else. And the Mission Control Preferences have nothing to do with this.