Fatal error! I can't open the web site anymore

you have two options remove the get_query_template() function from theme.php file. It is isn’t there by default so you don’t need it there. Just go the theme.php file and remove the function which looks similar to this :-

function get_query_template( $type, $templates = array() ) {
$type = preg_replace( ‘|[^a-z0-9-]+|’, ”, $type );

if ( empty( $templates ) )
$templates = array(“{$type}.php”);

return apply_filters( “{$type}_template”, locate_template( $templates ) );
`}

or just download wordpress again and restore just the wp-includes folder.

Thank you very much sk_tamilan I restored the wp-includes folder and it works again!

No sorry I can not enter the wp-admin page!

Here is the message:
Fatal error: Cannot redeclare win_is_writable() (previously declared in /web/htdocs/www.xardinia.com/home/wp-includes/functions.php:1448) in /web/htdocs/www.xardinia.com/home/wp-admin/includes/misc.php on line 557

Do I need to restore also the wp-admin folder? I’m nor sure if I changed something in there (Im not working on this website since long) and I fear I will loose some infos.

What should I do?

you can restore pretty much anything from the original wordpress except for the wp-content folder as thats the folder which contains all your plugins, themes, uploads.

I strongly recommend not to play around with the other wordpress core folders! you don’t need to be editing anything in wp-admin, wp-content and wp-includes.

if you are customising your themes its all in wp-content

I restored wp-admin but it still gives the same error message:

Fatal error: Cannot redeclare win_is_writable() (previously declared in /web/htdocs/www.xardinia.com/home/wp-includes/functions.php:1448) in /web/htdocs/www.xardinia.com/home/wp-admin/includes/misc.php on line 557

Any idea?

Try re-uploading all files & folders – except the wp-content folder – from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones.

Help! I re-uploaded all files & folders – except the wp-content folder – from a fresh download of WordPress as you told, but now it doesn’ t load the site anymore!!

Try checking your site’s error logs for a more specific error message. Your hosts should be able to help you in accessing your site’s error logs.

You are maybe getting a database connection error?

there might be a chance if you replaced the wp-config.php
or if its no longer there due to the new files you copied then i hope you have a backup of this file or you will have to set this up again – just open the file up and modify the fields.

Sk_tamilan you were right! I’m so terribly a beginner, I replaced wp-config.php without first modify the fields. I still had the old one saved somewhere, now it works!
Thank you very much to both of you!