How to retrieve deleted Jenkins job?

It is not possible unless you somehow restore files from your master operating system. In future you can use plugin: https://wiki.jenkins-ci.org/display/JENKINS/JobConfigHistory+Plugin. This plugin saves configuration files for deleted jobs (but not build history).

In fact you have just to use this url to reactivate your deleted job : http://[url_of_jenkins]/jobConfigHistory/?filter=deleted

You just need to have this plugin installed beforehand: https://wiki.jenkins.io/display/JENKINS/JobConfigHistory+Plugin