I have had several instances where MySQL fails to start after a power failure or other improper shutdown.

This has only happen on FreePBX servers for me, but those are really the only systems I have with MySQL that aren’t in a controlled environment and I assume this can happen in other situations too.

On FreePBX this has the interesting effect that the underlying Asterisk system works perfectly, but the FreePBX configuration GUI is completely only shows the error message:

FATAL ERROR

DB Error: connect failed

 The problem seems to be that the file containing the UNIX socket is not deleted and prevents the server from starting correctly. This is easily fixed with:

mv /var/lib/mysql/mysql.sock /var/lib/mysql/mysql.sock.bak
service mysqld start