I’m having this same problem suddenly on 2 blogs (https://e9hh.com/blog and https://www.xplosiveworld.com).
Both sites seem to be experiencing the same problem and pulling the Error establishing a database connection message.
The only thing I can think of that might be causing this is I redirected both sites to pull the URL’s starting with the www prefix in the settings, but both worked fine after making the change.
Help is much appreciated!