Decrease of traffic?

If your main traffic was from Google, they simply changed their algorithms which you cant do anything about but wait. Don't panic though if you had traffic it will come back as their spiders find your site again. My suggestion is to just keep adding good content and they will come and spider your site again and all will be back to normal. This happened to my site as well. Inbound links help a ton. Maybe resubmit to Google again @ http://www.google.com/addurl/

first of all, we need to drill down the problem:
1) which keywords people usually used to refer to your website?
2) what is the ranking of your website through these keywords?

mostly people will click to the site listed on the first 3 SERP (search engine result page). it is needed to monitor your keyword ranking. if there is a change in the algorithm of the search engine, your ranking could go up, or down, which will ultimately affect your traffic coming from search engine.

Search engine optimization (SEO) is your starting point for getting traffic. All SEO means is making your pages human and Google friendly. Your website needs seo friendly search engines.

The first criterion (often overlooked) is to have great content! Your goal is to build traffic. Why would anyone even want to come to your website if there's nothing interesting there to see?

Your pages need to be built around specific keyword phrases that people use to find sites like yours. So you have to do some keyword work on your site.

updates are important. Add new pages to your site with good content.