How to complain a freelancer?

Hi,

Does anyone know how to complain a freelancer? We hired a freelancer to update a very small function of our website. But he completed messed it up that our website is not working. I have to get other developers to fix it. 

I don't know what the developer did to our platform. But it was working perfectly fine before he access to it. 

Appreciate if anyone share your experience how to complain the freelancer. This kind of not qualified freelancer shouldn't be on the platform at all.