This is the wrong forum section, but your answer is simple.

You don't have enough decent links to keep a good ranking.

1/2 of your incoming links are nofollow also.

It's a new site started in March so if you had good rankings it was probably just the new site honeymoon period and now the site is where it belongs - you need more links.

I'm guessing that's why you used a keyword link in your post, but you need more.

Andyhenry - You are exactly right

Guptaarun - Some high quality backlinks will fix your problem

I think you should invest in some good Dofollow SEO links and vary the keywords. It seems that google finally caught up to you.

The biggest factor that influence your site rank is quality backlinks. Just like other said you need to build quality backlinks with your keyword as anchor text.

Andyhenry is right.

Another reason that can cause your ranking to drop is, your competitors are building or have built better links than you for that keyword.

do some link exchange with your niche related websites ..

You should check backlinks of your website for this keyword.Perhaps your website lost some quality backlinks pointing to this keyword.So build some more quality and relevant backlinks to get back your keyword's ranking.

I see that almost everyone is recommending link building to help you keep high rankings.
My advice is to be careful with what types of backlinks you create, the quality of the sources, link velocity and anchor text distribution.
Your focus should be on making the process look natural for Google and other search engines.