I just want the equivalent of closing and reopening my main program. I want to invoke it when a "new"-like option from a drop-down menu is clicked on. Something like calling root.destroy() and then re-initiating the mainloop.
How can I get this done?
There are at least three ways you can solve this. 
Method one: the head fake. When you create your app, don't put all the widgets in the root window. Instead, hide the root window and create a new toplevel that represents your application. When you restart it's just a matter of destroying that new toplevel and re-running all your start-up logic.
Method two: nuke and pave. Similar in concept but slightly different in execution. In this model, when you want to restart you simply delete all the widgets in the main window, reset the geometry to null (so the window will once again resize itself based on its contents) and then run the logic that draws all the other widgets.
Method three: if it worked the first time... As suggested by Martin v. Löwis, simply have your program exec a new instance of the program, then exit. 
The first two methods are potentially faster and have the (dis?)advantage of preserving the current environment. For example you could save the copy of the clipboard, column widths, etc. The third method absolutely guarantees a blank slate.
Resetting the main GUI window