Hi,
To solve this you will have to connect using an FTP client to your site.
You should then download the functions.php file from your theme and remove the modifications you've done before uploading it again.
When making modifications to existing themes you should use what is known as a Child Theme (You can read more about them at http://codex.wordpress.org/Child_Themes) which is a better way to go about making adjustments to a theme.