More like a flicker I should say, and then Photoshop seems to have hung. Well apparently the window is still there, its just off screen. Press ‘esc’ to close it and try File->save again. If it still doesn’t show up, use the following combination:
Alt key, Space bar, ‘M’
and use the arrow keys to guide the lost dialog back to your screen. If pressing left doesn’t work, try pressing right. Have fun.
For more details: