Ask HN: Website suddenly went down in ranking?

Any SEOs ninjas here please? my site www.agicent.com, which was doing good for the keyword app development company (ranking between 1st to 3rd page on google since a month) suddenly pushed back to 11th page, and the traffic reduced drastically as well.

I thoroughly checked the site and figured no bad backlink, though I got major crawl errors and duplicate pages (my original pages were duplicated under the agicent.com/blog/ root folder as well FYI), so I fixed that and resubmitted the site map.

Can this be the reason of that sudden decline (although google says 404 errors don't harm rankings)?

Can anyone please help me analysing the issue, and possibly suggest a fix.

Thanks,