It has been broken for me too,
I usually rely on google anyway, using chess.com as a search term.
This has been discussed before, as can be seen on the following page:
http://www.chess.com/forum/search.html?keyword=site+search+not+working
If you need help, please contact our Help and Support team.
I've tried the 'site search' several times over a week or so, and not once has it worked (for me, anyway). Is it meant to be functional?
sftac