So, while using the Construction Set, I accidentally double clicked on the main window's icon which closed it. Now, whenever I open the Construction Set, that main, essential, window doesn't show up, but the three others do. How do I fix this..?!

You most likely need to maximize that window again.

Nevermind - I'm a stupid. If anyone comes across this issue, just simply move away the other tabs, and you'll most likely find it.