500 Internal Error

Error 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.
– 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 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.

In 90% of cases such kind of problem can be solved by client and does not need Administrator rights.
Usually the error “500 Internal Server Error” appears when:

.php files have incorrect permissions. For example , if you try to run PHP file with 777 permissions or PHP file is located in the folder with 777 permission, then “500 Internal Server Error” will be generated. Please check the article “Permissions for *.php files” for more info.
.htaccess file has incorrect directives. For example, when someone tries to use .htaccess file to change PHP variables like:
php_value register_globals On
For this purpose you should use o n l y php.ini file. Anyway If you are not sure what can be wrong in .htaccess file just try to access your php script without .htaccess (temporary remove or rename it ).

I agree with WebHosting Watch: it can be permissions…but I’m not sure I fully agree with that it’s usually the problem.

In my opinion, please don’t just change things hoping they will work. That’s shooting in the dark, which isn’t good. It’s just bad practice.

Find your PHP or apache error log, and look for the error in there. Then you’ll know what course of action to take.

If nothing is there, you’ll need to adjust the logging on your server. Take a look here
https://php.net/manual/en/errorfunc.configuration.php

Best of luck!