I got it. This phenomenon occurs when access to your site becomes excessive. I guess it is caused by MySQL connection limit. Please look a little more appearance. And, if error occurs frequently, you should move the server or database. There is a method of changing the entire server. But, there is also a method to rent a another server for the only database. In addition, because allocated memory of cheap VPS is small, there is an excessive process is forcibly killed. If you have a budget, I recommend dedicated server or full managed dedicated server.