This tends to happen from time to time when improper system shutdowns occur. Mysql will refuse to start up due to Another MySQL daemon already running with the same unix socket.
To fix this up, real simple, just go to /var/lib/mysql and rename the file mysql.sock to something else.
mv mysql.sock mysql.sock.orig
Start up mysql again...
service mysql start (mysql for Percona variant)
There you go, Mysql has started up :)