Correst a bug on a form in PRESTASHOP

Hi all
I have a website. It is made with prestashop.
It works fine but there is an issue in a form.
It used to work but it doesn't anymore.
I want someone to fix this AND TELL/SHOW ME HOW HE/SHE DID IT?
Not a big thing to do, Fast done, fast paid.
Do not bif too much, I won't have time to answer.
Regards, Christian
CMS, HTML, PHP, Prestashop