If your SERPs have suddenly dropped, you've run afoul of Google. If that's the case, try to figure out what you've done wrong and fix it. Then keep on link building slowly and steadily. The absolute worst case scenario is that they remove you from their index. Do a Google search on your site's domain name. If you don't come up at all, then you've been removed.
It could be worth checking the Google toolbar. If your page rank has been greyed out, it could mean your site has been blacklisted.