500 - Internal server error (WordPress)
This problem can happen, if your server host has its timeout settings on too low, so you should contact your server host, and ask them to help resolve it. There are other two possible errors, but it's 99% that the server will be at fault.
You could try to increase the memory limit in your wp-config.php file by adding this code to it, because that is one of the possible causes:
You have a plugin with an error
If you can't access your whole back-end, then you should open your ftp, and rename the plugin folder names in your wp-content\plugins\ folder one-by-one to see, if one of them solves your problem. If you only see the internal server error at the Smart Slider's page, then you can deactivate your plugins in your Plugins list too.
Bad .htaccess file
Go to your ftp into the root folder, and rename your .htaccess file to something else to see, if it solves the problem. If it does, you can keep it like that, this file is to modify the server's configuration.
If none of the above would help, you could check out what kind of PHP error is triggering the server error like this.