Can't access my code igniter 3 website

In application/config/config.php file set $config['log_threshold'] = 1;

Make sure, the application/logs directory is writable.

Disable .htaccess file by renaming it to htaccess (remove dot from the beginning).

Try to access pages with index.php in the URL. For example: site.com/index.php/controller/method

If it is working with index.php, then the problem is in .htaccess rules. There is no universal solution for this and you must play with it. Every shared hosting is different.

Rename htaccess file back to .htaccess

If it is not working at all, then look in the log files in application/logs directory for more information. There should be some.