Reopen Oscilloscope Window in Proteus 8
In order to show again the closed oscilloscope window start the simulation, then click pause and open the debug menu. From there click the oscilloscope option and the oscilloscope window will appear.
You can then resume or stop and restart the simulation and the window will be visible. The same stands for any other closed debug window.
Using the Reset Debug Popup Windows option in the debug menu (enabled only when the simulation is stopped) has a similar effect but the difference is that it will show all closed windows while with the way I describe you can show the selected windows only.