If you have selected the Order Item icon (the shopping cart) and the Place Order Window does not appear, then this may be because you have your computer setup with two screens but one is not plugged in.  ProSelect remembers if you moved the Place Order window to the other screen and will place it there the next time you open it.

Of course, if you don't have that screen plugged in, you won't see it!

Hint:
This can also occur with other ProSelect windows such as your Preferences, Templates Editor etc.