To periodically check status of the cluster, create a script (/tmp/xx):

#!/bin/sh
mysql --defaults-file=/path/to/.my.cnf -s -e "SHOW GLOBAL STATUS WHERE Variable_name IN ('wsrep_ready', 'wsrep_cluster_size', 'wsrep_cluster_status', 'wsrep_connected');"

And run it with:

watch sh /tmp/xx

If single node alive (ERROR 1047 WSREP has not yet prepared node for application use):

SET GLOBAL wsrep_provider_options='pc.bootstrap=YES';

This node can be used now has the new master so others nodes can recover from it.

grastate.dat

Try to recover from the node with max seqno, for example this is a node that shows a graceful shutdown:

version: 2.1
uuid: cbd332a9-f617-11e2-b77d-3ee9fa637069
seqno: 43760

This grastate.dat file shows -1 in the seqno. This node crashed during transaction processing:

version: 2.1
uuid: cbd332a9-f617-11e2-b77d-3ee9fa637069
seqno: -1

This node crashed during DDL:

version: 2.1
uuid: 00000000-0000-0000-0000-000000000000
seqno: -1