Hi there,
Others have already mentioned great starting points. The steps I would take here in order would look something like:
- Confirm that I'm looking at the right data, and identify where the drop is coming from. Is it mostly the blog? A certain section? Top performing pages? Homepage? Or is it more of an all around drop to most pages?
- Once you answer the first question, it will allow you to prioritize where to look. If you identify most of the drop happened on your blog for example, you can focus your attention there to answer questions like, "Have we changed anything on the blog recently from a technical perspective?", "Are we confident there is nothing that is stopping us from ranking technically?", etc.
- The more difficult situation is when there is a general decrease in traffic to most pages on the site (doesn't seem to be a rhyme or a reason). In these cases I would look back at what updates have affected the site before to see a trend. When did the drop happen? Does that drop line up with a recent update? If so, what was that update about? Then some form of a technical audit, content audit, etc would be reasonable next steps to essentially identify the biggest issues in each.
Some other points in no particular order
- Remember that more content/pages does not necessarily mean more traffic. Because your formula was working a year ago doesn't mean it will work now (or work as well).
- Looking at your site briefly, you have a good amount of content (~17,000 pages) but it's difficult to navigate and find articles. If I had to guess, I'd say the site could benefit from a re-design and content overhaul/audit to ensure there are not too many overlapping pages and that they are easily discoverable by users (and crawlers).
I know that's a bit of a brain dump but hope that (along with other response) helps point you to a starting point!