During the time that SketchUp wasn’t responding, did it recover if you pressed the Escape key?
If it did, it could be that the window was off screen. The way to solve that is to do these steps, where ‘M’ is the key that represents Move. If your Windows is not English that key may be a different one:
Open the Add More Imagery window, even if you don’t see it. Hold down Alt, and tap the spacebar. Release Alt, and tap the M key. Tap any arrow key. Now move your cursor, and see if the add location window is attached. Left-click to drop the window off where you want it to be located.