The reason why you cannot see the window of the BOSS POS even if you opened it's icon is because it is minimized.

You will notice that as you click the BOSS POS icon, a little line is displayed somewhere on your screen. See below sample:

In order to maximize the BOSS POS window:

1. Right click the BOSS POS icon on your computer's taskbar.

2. Click "More Actions".

3.  Check the "Maximize" checkbox.

Now the BOSS POS window shall be displayed on your screen.