Couldn't connect to server 127.0.0.1:27017

Step 1: Remove lock file.
sudo rm /var/lib/mongodb/mongod.lock

Step 2: Repair mongodb. 
sudo mongod --repair 

Step 3: start mongodb.
sudo start mongodb 
or
sudo service mongodb start

Step 4: Check status of mongodb.
sudo status mongodb 
or   
sudo service mongodb status

Step 5: Start mongo console.
mongo 

Did you run mongod before running mongo?

I followed installation instructions for mongodb from http://docs.mongodb.org/manual/tutorial/install-mongodb-on-os-x/ and I had the same error as you only when I ran mongo before actually running the mongo process with mongod. I thought installing mongodb would also launch it but you need to launch it manually with mongod before you do anything else that needs mongodb.

This it's because the mongod process it's down, you must to run the commands bellow in order to get up the mongod process:

~$ sudo service mongodb stop

~$ sudo rm /var/lib/mongodb/mongod.lock

~$ sudo mongod --repair --dbpath /var/lib/mongodb

~$ sudo mongod --fork --logpath /var/lib/mongodb/mongodb.log --dbpath /var/lib/mongodb

~$ sudo service mongodb start

Hope this helps you.