Comment on WordPress SEO Plugin Options by SEO Dave.
If the site works with the standard WordPress theme it would suggest an incompatibility with Stallion.
If you are currently locked out of the sites admin screen you’ll need to delete Stallion using FTP or your hosts control panel. Go to /wp-content/themes/ and delete the /stallion-seo-theme/ folder. When an active theme is deleted this way WordPress reverts the site back to the default theme which will give you access to the WordPress Dashboard.
Make sure you have the default WordPress theme within the /wp-content/themes/ folder otherwise there won’t be a theme to default to.
There’s a lot of code snippets and plugins added to Stallion and it can cause issues with other plugins that are running. On this site I used to run a plugin that would randomly ‘crash’ the site (white screen) so had to stop using it.
First step is check your error logs or turn on error reporting in your wp_config.php file or run a plugin like https://wordpress.org/plugins/log-deprecated-notices/ that might store the errors before the error takes the site down.
If you can get the error messages we might be able to track down the cause.