Basically your Keyword ranking goes down. This is because lesser link submissions or SEO is not performing that effectively. You need to work well on both on Page and off page optimization. For Off page you need to work on directories and social bookmarking and also do the audit of your on page social optimization like analyzing Keywords density
You need to build continuous links to maintain ranking of your site, there would be competitions and competitors are doing their works so you need to do too.
Make your strategy more diverse. Create more backlinks, create some videos with your link in them, some facebook pages, etc..
It's great to hear that you got into the top 10 in the first 15 days! What techniques did you use? How did you give it momentum, create link popularity etc? I'd like to know your techniques!
What happened is Google punished you because you were probably using Black Hat SEO (possibly by accident without knowing you were using a banned technique). When in doubt use the SEO Checklist. It contains a list of what is good White Hat SEO and what things are banned Black Hat SEO.
quality content always plays a vital role in your web's life . update your website frequently and make as many back link as you can to give high search result . and it should be slow and steady work . dean
Check and see if you got hit with a google penalty...this happened to one of my friends sites a few months ago and it was devastating.