If you have not made any changes to your database, or mainfile.php and yet suddenly see this message instaed of your website, the probability is that your database is either down or compromised. If you are on a hosted server, check their status before investigating your files. This error may occur for a short time period during upgrades and maintenance as well as genuine faults.
Or the technical peeps at the host company have fixed it !
Looks to me like there was a temporary connection issue to the database server.
I work for an online company as a Systems/DB administrator and it makes me scream at the screen when we have a problem, fix it quickly, and the people in the chat room put it down to the site correcting itself !!
Sites do not correct themselves. If they did we would all be out of a job
I'm sure if you email the hosts they will be more than happy to provide some kind of explanation.
When I tried to get on to the main site today it half loaded and then came up with an adult dating site. It did this three times before I finally got on to the main. I've got mega blockers on this 'puter so I know it's not a pop-up just overlaying the main page. The main site header came up with the BP logo etc as usual and then was wholly replaced by the dating site one.
Sorry, couldn't get a screen grab of it as I don't know how to.