This is a Server error, so you will need to contact your Hosting provider in order to fix this issue. They may have to revert back to a previously saved point.
Also, I always recommend making edits to the Stylesheet, Functions file and any other file through FTP. If you make an error on a php file through the WordPress editor, there is no easy way to go back, especially if it takes down your whole site when it shows the PHP error. If you use an FTP program and make a mistake, your site will still go down, but all you would have to do is erase or undo the mistake to bring it back up instantly. You can do a Google search on how to setup and use FTP since there are plenty of tutorials available. Make sure and find one that is specific to your host.