Don't beat yourself up; it’s probably a case of the dialog box slipping off your screen, which can happen for several reasons. Perhaps you changed your screen resolution, or maybe you typically use dual monitors, and now you have only one. Either way, it’s a fairly simple fix.
First - change your screen resolution to its highest setting and see if you can see the dialog. It could be that it is near the edge of the screen and is not displayable at lower resolutions. If you see it, drag it back to the center of the screen and return your resolution to its previous setting.
If changing the resolution does not resolve the problem, here is a simple method to get it back.
That's it. The next time a dialog box goes A.W.O.L., don't get frustrated. Follow either of these simple methods, and you'll be back to work in no time.