You said that the site was HTML before the change right ?
Let’s say you had a page called ‘random-casino-review’ – How the url was look like ?
/random-casino-review.html ?
If so it looked that you didn’t save the old URL structure … Now Google is looking for the old HTML files and get 404 … Let’s take this old HTML url for example : bonusblackjack.net/vegas-red-casino.html …. Currently it’s returning 404 !!!! – And their are bunch of other pages that return 404 error !!!
You should have kept the old urls structure .
I guess that Google is now reevaluating the site.