iPhone 6s plus screen broken problem

First of all check that the phone itself still works, a popular method is to plug it in to a computer and listen for any tones, or call it from another phone. If it still works then all that is needed is a screen replacement.

Buy a new screen from here:

This is the screen by itself

iPhone 6s Plus LCD and Digitizer - Original LCD

This is the screen plus the tools you'll need to do the repair

[prodotto collegato mancante o disabilitato: IF315-033-7]

And this is the guide you'll need to follow in order to replace the part.

Sostituzione dello schermo dell'iPhone 6s Plus

Let me know if this fixes your issue.