Error 500 ISE is very generic and can be caused by numerous underlying issues. Your first step should be locating error log for your hosting account or asking support to help with that.
– It may indicate .htaccess issue. Regenerate .htaccess, and be sure the encoding in your text editor is Unicode No BOM
-It may also indicate a problem with the configuration on the server. Usually your hosting company tech support can help.
– it could be problem with the plugins. try resetting the plugins folder by FTP or phpMyAdmin.
– it could be a problem with the theme. Momentarily, try switching to the default twentyeleven theme via FTP by renaming your current theme to rule out any theme specific issue.
– if these steps don’t help, download WordPress again and replace your copies of everything EXCEPT the wp-config.php file and the /wp-content/ directory with fresh copies. This should replace your core files without changing your content and settings in wp-config.php file and the /wp-content/ directory.
– did you check with your host if the domain name is configured properly to point to public_html folder