New domain vanishes? What can I do?

I created a new domain in June 14, in August/Sept I created a PBN and the site ranked in top 3, in Feb/March 15, I noticed an old business associate had created a rival site and targeting the same keywords!

So I started building tier 2 & 3 links to the PBN using socialadr and followlike (Facebook/Twitter/Google+), now the home page has fallen off google (not in top 10 pages after being at #3 spot for months!!!!! All other sites in PBN still ranking strong, no issues.

Inner page still on page 1 - home page still indexed in google. I am checking using serpbook and discovered this yesterday.

All content unique and of quality - not spun! Any ideas what could have gone wrong? Is it google dancing?? What can I do to come back??

Thanks.

Have a look at the backlinks created in the last 30days? may be someone is doing Neg. seo to your site?

Thanks for that, just checked no Neg SEO up to now...Would getting some guest posts on high DA to the url 'push' it back up?

Feb/March could have been Google's mobile algorithm update for their search engine, which might affect your serp