Sounds like a "whitespace" problem. Make sure there aren't any extra spaces after the code on line 18 of functions.php.
If it's an error in the theme, then you can temporarily delete or rename the current theme folder. WP should then automatically switch back to the default theme and let you log in.
Maybe its some file permissions error. Check the theme's read me or something and see if there is anything that needs to be modified.
I would definitely start by doing this. If you have recently updated your WordPress version, it could be that your theme is broken..so go into your filemanager in your hosting account and do what Kerosene suggests. Good luck!