it's actually very normal. Many fresh sites get traffic during the first 1-4 weeks of going live, and then they disappear from the SERPs. Why this happens I do not know, I can only speculate (google putting your content in front of their users to get the initial feedback based on which your site will stabilize somewhere in top 100 if it's good site, or nowhere if it's bad site as google themselves deem your site as being good or bad, not you or your users, which sometimes it's unfair but it's google's game, so it's google's rules....)
Just keep working on your site like nothing happened and if you do the right things (keep publishing quality content like a robot, day in day out, interlinking the content, building links and traffic from other sources, etc) you have a high chance of starting to rank in the next 3-9 months.... unless you target really tough keywords, of course. In that case you might rank in the next 2 years or never, depending on google's whims and the strength of your competition....
Also, note that for some keywords you can never rank as for some keywords (the best ones most of the time) google keeps the top 3 or top 5 positions reserved to the ultra rich of the rich, so you can't take those guys down no matter what you do.... or, if they're not rich, then google has an interest that we don't know about, to keep them ranked up top all the time. This is a rule that's hard coded into google's algorithm, so you literally can't rank in top 3 or top 5 for some keywords, which is why I said that you need to target the right keywords.
Anyway, I didn't intend to put you down (morally I mean), I only wanted to tell you what you can expect in today's SEO, part of which the de-ranking of your site after the initial rankings is very normal, like I said...
