Admin area not showing

I recently upgraded to 3.1 and ever since then I’m encountering all kinds of problems, so I’m fixing them as I’m finding them. website looks fine, but admin area is lost.
This is what I get:
Warning: Cannot modify header information – headers already sent by (output started at /homepages/26/d301038739/htdocs/nolan/wp-includes/functions.php:2) in /homepages/26/d301038739/htdocs/nolan/wp-includes/pluggable.php on line 897

Any help would be greatly appreciated.