Error 500 ISE is very generic and can be caused by numerous underlying issues. Here are some thing you can try to solve it:
- Your first step should be checking your site's error logs for a more specific error message. If you need help locating them, ask your hosting provider to help you with that.
- check with your host if the domain name is configured properly to point to public_html folder
- 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 WordPress theme (TWENTY TWELVE) via FTP or whatever file management application your host provides by renaming your current theme to rule out any theme specific issue.
- if these steps don't help, download WordPress again and manually 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. Read the Manual Update directions first!