MySQL server (MariaDB) cannot start

I'm trying to start MySQL server (Maria DB 10.0.12, installed via Homebrew) but it's not working.

When I run mysql.server start, it keeps running forever without actually starting the server.

mysql.server configtest tells me the config is fine and there is nothing in MySQL error log.

I haven't actually changed anything to MySQL recently, though I've upgraded PHP and other modules (but I guess that shouldn't be related).

Any idea what could be causing this, or how to debug it?

One place to start debugging this is to check for a err file. It will be named with the hostname of your Mac and you should find it in /usr/local/var/mysql. So if you Mac is called charles the file should be /usr/local/var/mysql/charles.err.