WSREP will block mysql clients when the sum of nodes below quorum.

ERROR 1047 (08S01): WSREP has not yet prepared node for application use

This is true with two nodes one garb. To set the remaining surviving node as master:

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

To force the surviving node accept connections:

SET GLOBAL wsrep_provider_options='pc.ignore_sb=TRUE';

Warning: Enabling pc.ignore_sb is dangerous in a multi-master setup, due to the aforementioned risk for split-brain situations. However, it does simplify things in master-slave clusters, (especially in cases where you only use two nodes).