one of the nodes failed then the whole cluster went down.
shutdown mysql on all nodes
cat /var/lib/mysql/grastate.dat
check which node has the highest seq number
boot strap that node
service mysql bootstrap-pxc
then start the next node but before that
mv /var/lib/mysql/grastate.dat ~
service mysql start
this will start a full resync
wait till it finishes then start the same process on the next node.
https://www.percona.com/doc/percona-xtradb-cluster/5.5/manual/bootstrap.html