Sorry to hear of the problem you are having.
This plugin internally uses Smush.It service provided by Yahoo by calling its APIs so the functionality of it depends on the Yahoo Smush.It service. If Yahoo have any service issues on their side this will also affect your site in how it smushes, this could be a cause of the error "Service Temporarily Unavailable".
You can try getting website back up and running by deactivating WP Smush.it plugin by renaming the following WP Smush.it plugin folder using FTP or any file manager.
If it doesn't help you then could you please share me your site URL or the detailed error message if any so that i can troubleshoot it?
To get detailed error message can you please try enabling WP_DEBUG mode? To do this, just go to wp-config.php include this:
You might already have a line for WP_DEBUG, so just make sure it's set to true.
Let me know what errors if any you are getting!