If so there are a few things you can do to help get you back where you need to be.
First let's look at some On-Site Factors:
It is very important that your web pages contain no spam. Sometimes when making changes to your site you have changed something on your pages that might be considered spam.
Make sure that your web site doesn't use hidden text, keyword stuffing, cloaking or too many redirects. Search engines don't like it and visitors are becoming keener on what happens in front of them. They don't like it either.
If you have new pages that aren't listed, make sure that you have enough text. Search engines need text to index web pages. Overusing frames can cause your information to be passed over by spiders.
Google(as well as every other major search engine) has difficulty indexing dynamically created pages. If you must use dynamically created pages and if your web page URLs contain questions marks, the & symbol and other special characters, consider rewriting your URLs so that your pages appear as static pages.
Make sure that your robots.txt file allows search engines to index your web site. A broken robots.txt file can send search engine spiders away from your pages.
You should also make sure that your web pages have valid HTML code. While most HTML errors don't cause problems, some of them can prevent search engine spiders from indexing your site. Validation is very helpful in this case.
Next time: Off-Site Factors and how they affect your rankings.