Sudden drop in rankings, don’t undertstand why

Recently I came back to my site that I didn’t do any SEO for since a few years. First thing I noticed is that Google was no longer considering it mobile friendly. So I updated the mobile template to bring the site up to the Google requirements. Also I registered a trial account with one Google rank checking tool. That’s all what I did. And right after I did that, around October 20, my site’s rankings have dropped for all important keywords and for all platforms (desktop, mobile, tablet). This resulted in a significant visitor drop as seen in the graph below.
It can’t be a coincidence but I am really baffled. I don’t see any reason why the rankings could have dropped because of what I did. I am even getting a weird idea that the Google ranking service might have done something to force me to buy their paid subscription. I know that sounds weird but really I can’t find a more logical explanation.
Does anybody have an idea why this might have happened?

Too many requests from Google resulted in my site being blocked

Hi everyone,
Yesterday Google made so many requests to my site that it exceeded the daily quota for script executions. As a result the service was suspended. I have checked Google Crawl Stats and it appears that since 1 February Google has been crawling my site in this mad way (yesterday’s figure is not yet seen on this plot, it must be even higher). As the host’s Tech Support suggested, I’ve limited the Google crawl rate. But that will come in effect only in 2 days, meanwhile Google still keeps crawling my site like that. Even if that will help after 2 days, why should Google crawl my site like that? Does anybody have any idea? Is there anything else I can do?