I started a site in mid-January (domain registered around January 12, site indexed around January 15) and since then, I've been creating a lot of content on it and doing some very aggressive link building (white hat only). The main keyword I was targeting gets more than 300,000 exact searches/month. In late February, the site's homepage was ranking on page 6 for that keyword but on around March 1, it completely disappeared from the SERPs (not in the top 100 or top 1000 or anywhere at all). One of the site's subpages though is ranking on page 43.
Is my site in the infamous sandbox, implying some sort of a domain age "penalty" and should I just wait it out while continuing to build links? Or is there something else I should be worried about?