What to do if Butterfly pupa fall down?

You can try to pin the top part of the pupa to a cotton ball and then attach that somewhere. YOU MUST BE VERY CAREFUL. If you leave it like that, its wings might become crippled. Good Luck.

http://www.butterflyfunfacts.com/reattach-swallowtail.php

Go to this website for even more info.

It depends on what kind of butterfly you have. If you have "The Painted Lady," you can simply lay the chysalis/pupa on a napkin.