1. The search engines have changed their algorithms. The search engines are constantly changing and improving their "ranking rules". Most changes are hardly noticeable, but every once in a while the changes are huge and will have a major impact on your rankings. It's important to keep tabs on the top search engines and optimize your pages accordingly.

2. There are technical problems with your website. If your site is down when the search engine spiders come a-knockin' they won't be able to crawl your pages and the search engines may remove your pages from their caches. Some technical issues might go unnoticed since your site may display the correct page in a browser while direct access to the server actually returns an error code. Ensure that your site is hosted on a reliable server and, if you suspect any problems, use a third party monitoring service to track your up time.

3. You've made too many changes to your website. While it's true that search engines love fresh, updated content, too many changes can actually hurt your rankings. According to Google, significant changes "may indicate that the document has changed owners and previous document indicators such as score, anchor text, etc are no longer reliable." If your site already enjoys high rankings, be very cautious about making any major changes.

4. Your competitors have updated their pages. It might not be that your page has "fallen" but rather, your competitor's page has risen. If they offer better content, better inbound links, better keyword density and better relevancy, it's only natural that they will earn a better ranking. Make it a habit to "spy" on your competitors and react to any changes in their rankings.

5. Your linking strategy is flawed. Quality outbound links are important to ensuring a good search engine ranking. But it's important that you use a solid, acceptable linking strategy. Buying links in order to increase your inbound links can get your site penalized by Google. Likewise, too many inbound links built up too quickly can throw up a red flag with Google. Your linking strategy should be one of building up inbound links from relevant pages slowly and consistently over time.

If you notice a sudden drop in your search engine rankings, don't panic. Just review the above list, narrow down the reason or reasons for your decline and make the necessary changes.