Elasticsearch service does not run at statup on Ubuntu 16.04
The answer to your problem is running this:
sudo systemctl enable elasticsearch.serviceElasticsearch on Ubuntu 16.04 uses SystemD not System V anymore.
For some reason when (as of 9th Jan 2016) you install ES 5.1 (not sure if other versions are affected) from APT by default this service does not start.
Did not find the error. Just reinstalled it with dpkg. Works, and the data and config hasn`t been overwritten.