You can enable the debug mode of WordPress to see what error you’re getting. This link can help you : http://codex.wordpress.org/Debugging_in_WordPress
tgd
(@terrydaisydesignsca)
Are you using a child theme, with its own functions.php file? You may have added or deleted something as simple as a php delimiter, or other character. If you use a code editor such as Dreamweaver, Edge Code, or even Komodo Edit, TextMate, BBEdit for Mac users, or most other open source code editors that use code coloring and syntax warnings, you should be able to see what’s missing or left in by error. Good luck. If you post the content of your functions.php file, the community can have a look and see what’s wrong.