In a recent post, a user was using 2 monitors, and when he went back to one monitor (laptop) the dialog was off screen and caused a similar problem to what you are experiencing, were you using more monitors previously by any chance, if so, try pressing windows key+shift+left arrow key (or right arrow - it doesn't matter).
In case the dialog is behind the TC window, make TC smaller, before trying the edit material, this should allow you to see if the dialog is behind TC.
If the dialog is just gone 'off screen' that's more fiddly as you won't know which direction its off, I've read that changing the windows screen resolution can help, but probably won't if the screen res is already at max, If you try the Alt+Space, then M, then arrow keys I'd be careful, I'd press the left arrow 2 or 3 times, and if no dialog appears, I'd return it to where it started using the opposite arrow, I'd do this with each direction.
If the dialog is 'hidden' i.e. a fault within TC, you could try resetting the configuration (this won't work if its a window fault) tools - customise - optios - workspace, click load-from or default buttons.
All the above is just guesswork, as I haven't experienced the problem myself, so its just what I've read here or on the web..