Normally, when we want to start PXC (Percona XtraDB Cluster) on RHEL/CentOS 6 or older then that, we can simply start with init.d script or service command. i.e
shell> /etc/init.d/mysql start OR shell> /etc/init.d/mysql bootstrap-pxc
shell> service mysql start.
So now, we have to start mysql with systemctl command i.e
shell> systemctl start mysql
shell> systemctl start firstname.lastname@example.org
I would also suggest to read one very informative post by Przemek for recovering PXC cluster. http://www.percona.com/blog/2014/09/01/galera-replication-how-to-recover-a-pxc-cluster/