Internatl error because of site map

Hello,

On our website, we wanted to add the site map in different languages as the main version of our website is in English.

But it seems the person I hired to do it made a mistake and since yesterday is completely down. This website brings me clients so I can’t let it down, it’s my main revenue.
to log in, i need to : enterdomain.com/site_en/wp-admin by example; That’s not normal
and all the pages of the website except the main page; to have the website showing up, i need to place mywebsitedomain.com/site_en/menu/page..

The website has an internal error 500 since yesterday because of site map and some mistakes performed.
Could you help me to solve this, please?