"If you sold Widgets, and a Widget-buying customer walks into your store, can't find any Widgets on her own, and when she asks what aisle they're in you remain silent, would you fire yourself?"
Has your website got a site search box on it? No? Then you're missing a big opportunity. There are two types of people who will use it: compulsive searchers who will always pick this route over any form of "navigation"; and visitors who couldn't follow your navigation and expect search to be there as an alternative. You'll lose both sets if you don't have a search box. And there are loads of ways of adding one.
OK, so let's assume you're happy with your website's search box. Now it's time to read Tweaking Internal Site-Searches into Buying Opportunities on the Conversion Rate Experts blog, which says that "There are many ways that search result pages can be optimized to provide visitors with an easy means to find what they are looking for – or at least something that will keep the visitor moving through your site" and gives a few ideas to get you kicked off. Remember, people expect your site search to be as good as Google, so don't let them down.