I have a small website about a commercial product, with a description and tutorial. 2 days ago it was in position 11th in Google search results, without any kind of SEO optimization on my part. Today it's gone. Totally gone - not even in the first 200 results. It's still very high in bing.com and duckduckgo.com. The site is very on topic. It's hosted under domain Keyword.com, and it's about commercial product which addresses the Keyword. How can I know what happened?
If you haven't done any SEO nor have many links to your site, its position in search results is highly volatile and can massively change every time Google makes a small change to its algorithm. I believe Google recently rolled out another update of its "Panda" algorithm to promote higher-quality content so maybe you got hit by that.
I'd suggest trying to get some more links to your site and making sure your
<h1> tags contain your keyword somewhere. See also: What are the best ways to increase a site's position in Google?
You can't know " what happened" really. It can be algorithm-change, as DisgruntledGoat wrote already, it maybe just Bad Day, on which more competitors appeared.
You can name your site, maybe some SEO-gurus will find obvious weakness in it. But common rule with search-engine output ranking is "Sometimes rubbish happens"