Manual:DBError.php

DBConnectionError
This is the infamous error:

If you see this, the database is very unhappy, but at least MediaWiki is still alive and knows about it.

Cannot connect to database
I haven't solved this for myself yet, but you should look into it following the same way I did.
 * 1) Check out the apache2 error log. If you're on ubuntu this was at "/var/log/apache2/error.log" by default when I wrote this.
 * 2) In my case, I got the following error .... "MySQL server has gone away in" .... followed by .... Error reading result set's header in ......
 * 3) This was supposedly caused by connection problems. Search those two terms and you should see several different places where people address it. Good luck!