I am coming from riak and redis where I never had an issue with this services starting, or to interact.
This is a pervasive problem with mongo and am rather clueless
This was happening with me today and I resolved it in following way.
Machine: I'm using Windows 10 machine and downloaded the latest MongoDB - Community edition.
So, the problem was, I did not have C:\data\db created.
Without creating C:\data\db, I opened CMD terminal and started the database using mongod command at terminal
C:\YourInstallationPath\bin>mongod
And when I fired mongo command then I was getting the problem.
Twist, I created the necessary folders but still was getting the problem. And this is because mongo server was already running. To fix it, I fired up mongod command again, and it automatically referenced to C:\data\db.
Other users have suggested to add C:\data\db but did not talk about executing mongod again, which exactly solved my problem.