My Website Was number 1 now its not on the first page!?

Welcome to my world! Due to the amount of exploiting and constant tweaking that search engines make to their algorithms to improve the SERP's for the searcher. The trick to it is that no one knows exactly what they did if they did anything at all. It could be some other competitors that did something to better their rankings pushing you off the page. First off you should know which page of your website was ranking #1 because it was not your entire site. After you narrow down the page you have to go through your SEO step by step. Start with internal and external links, make sure all the pages you point to actually work. Than your title's, spelling, source code for the meta tags in the HTML.
It's really a lot of time and effort to continuously keep your pages on the first page results, otherwise their wouldn't be a need for SEO experts. So if you have a lot of competition, usually ecommerce sites, or competitve services you should invest some of your marketing dollars in either an on staff employee or company or freelance SEO to take care of your site for a monthly or hourly fee. For total Search Engine Optimization or even a complete content management system with SEO and Web Design you many different options and should prepare for the future.