Silvery - 6:56 pm on Jun 24, 2010 (gmt 0)
It sounds like you're talking about regular keyword search results as opposed to the "7-pack" of local search results that can appear within keyword search under Google's Universal Search. It also does not sound like you're talking about rankings within Google Maps itself.
Perhaps your site was impacted by the "Mayday Update" which many have identified, which resulted in a loss of rankings for long-tail queries for some sites which Google deemed of "lower quality":
If your pages have fallen in rankings by many places, such as being pushed off page one and back a few pages, you've likely been penalized for something you've done.
If your pages have only fallen a little, you'd need to look at a great many other factors which could have caused you to lose some of your ranking.
If you've made no changes recently which would result in loss of rankings, you may need to work on developing more external links to point to your site.