/sys/ is a symlink to /usr/src/sys/, I've had some occasions where it took off and went on a holiday, so make sure to check /usr/src/
It /usr/src doesn't exist or is empty, then you will need to fetch the source as roddierod mentioned.
The easiest way to do that is CVSup, see the corresponding chapter in the FreeBSD handbook for more info:
http://www.freebsd.org/doc/en_US.ISO...ook/cvsup.html