Before you do a Site Reconsideration which will have a Google employee looking very closely at your site the questions I would be asking myself include.
- Did the amount of content onpage change radically from before?
- Did internal navigation change radically?
- Is the overhaul causing a technical issue with Google crawling the site?
- Are rankings stable in other engines? Crawling and indexing ok in other engines?
- Have you I used the tool in WMT to view our site as googlebot views it.
- DNS ok?
- Server security still intact? Did the overhaul include a new script or app that opened the server to a new security vuln?
I experienced something similar with one of our sites that ranked for some incredible terms that took over a decade to rank for. After the redesign it took a couple months for the site to move back up to its original levels.
We changed a huge amount of onpage though and I suspect it may have looked suspicious to Google so it dampered us and ran the site through its systems again.
Also one thing to keep in mind is the huge changes taking place in Google right now. Perhaps this is just a coincidence?