MongoDB service not working after upgrade to Ubuntu 15.04
Since your error is:

Failed to restart mongod.service: Unit mongod.service failed to load: No such file or directory.


This is pretty much saying that your startup path is not in the expected location or else the Ubuntu install has led to problems. See: http://docs.mongodb.org/manual/tutorial/manage-mongodb-processes/

EDIT: Updated with link to what appears to be the answer, namely something is wrong with Ubuntu and mongo.  

https://stackoverflow.com/questions/29879231/upgrade-to-ubuntu-15-04-from-14-10-breaks-mongo-how-to-fix   

Joel Parke has defined the problem and pointed to a ticket on the issue. https://jira.mongodb.org/browse/SERVER-17742  

That ticket reportedly explains how to "cleanly install current versions of mongodb on Ubuntu 15.04 by replacing the apt source."