MariaDB Galera Cluster on Ubuntu 16.04 does not start after rebooting
This is likely to be a network or firewall-related issue. 

A few things to check:


Do you get the same error on both nodes? 
Have the IP addresses changed? 
Are the relevant ports open in the firewall? Relevant ports are 3306, 4567, 4568, 4444. See here for details: Galera Firewall Settings 
Could access to any of the ports be blocked by AppArmor?