DOCUMENT ROOT

The hosting does not have access to the site configuration to change the "DOCUMENT ROOT". 
Can I get it back like it was?
In the root index.php .
What should I do?

You could use a .htaccess file in the root to point to the /public/ folder

eg.

Code:

RewriteEngine on
RewriteCond %{HTTP_HOST} ^yourdomain.tld$ [NC,OR]
RewriteCond %{HTTP_HOST} ^www.yourdomain.tld$
RewriteCond %{REQUEST_URI} !public/
RewriteRule (.*) /public/$1 [L]

so that when you visit yourdomain.tld it will read out of the public folder

No. No. No.

Change the hoster or ask the hoster for help.