When you log into your WordPress site, you could come across the problem that plugin is not showing in the WordPress dashboard. Additionally, you notice that:
- Shield is inactive
- Shield is not showing in your list of installed plugins, active or inactive
- when you try to re-install it says the folder already exists
The reason for this could be that WordPress corrupted the plugin perhaps during an update, or your disk file system corrupted and it's broken it.
So, what you'll need to do is delete the plugin folder from your web hosting space (using FTP) and reinstall it. The settings will be saved already in your database, so that won't be a problem for you.
The folder you'll be looking for is /wp-content/plugins/wp-simple-firewall/