500 - Internal server error (WordPress)
500 errors are just "catch all" error messages, so they aren't telling you the cause, they are just telling you there is a problem and the server host has an error log, which contains the real error message. You should contact your server host and ask them for that log, also ask them to take a look at it, because a common server error is, that the memory_limit is reached and in that case your server host would be able to increase that value.
If it won't be a server limitation, but an actual PHP error, from the error message you will see which file causes the problem and you could try to eliminate the given plugin from your website or contact their developers to fix the issue!
Increase the memory_limit
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:
but server hosts usually won't give you control over server given values, so your code might won't do anything.
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
If you are editing your .htaccess file, so not the default WordPress one is used and you see this error, there is a chance, that you modified the .htaccess file wrongly. You should put back the original WordPress given .htaccess file to see if your problem disappears that way.