If website is not working even index.php contains only simple code like echo "abc";

if you changed index.php to die("its cool"); and the site still shows the same 500 error, then the request is probably not reaching that file.

1. Check document root in hosting
2. create simple html file and Check if .html executed or not
3. create simple php file and check if .php executed or not

1. If test.html gives 500: this is server/vhost config,
2. If HTML works but test.php gives 500: PHP handler/config issue in Plesk.
3. If both work but homepage gives 500: WordPress/plugin/theme/rewrite issue.
   
   how to fix
   1. check .user.ini if any auto_prepend_file in it, issue is in that file fix it. (user.ini contains configration of current folder like php.ini for whole engine)
   2.