Is it just me /
When I sign into the site I lose the search functionality. This is one both Chrome and Internet explorer.
No idea if it is just you.
The search form is contained in a element with the id block-search-0. You could check if your browser hides that for some reason, maybe due to a CSS rules.
I'd guess that at least Chrome should have an easy way to figure this out, maybe http://www.threetipsaday.com/2008/12/debug-inspect-google-chrome-inspector/