Couldn't connect to server 127.0.0.1:27017

Question

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.

The log indicates that mongodb is terminating because there is an old lock file.

If you are not and were not running with journaling, remove the lock file, run repair, and start mongodb again.

If you are or were running with journaling turned on, see the relevant Mongo DB docs. Note that they say "If you are running with Journaling you should not do a repair to recover to a consistent state." So if you were journaling, the repair may have made things worse.

Try

sudo service mongodb start

This solved my issue.

MAC OS

if you have mac and useing mongodb4.4, you can run:

brew services start [email protected]

Check your filesystem free space and increase it if its less. This also could cause the mongo not to start. Check the /var/log/mongodb/mongodb.log file.

ERROR: Insufficient free space for journal files
Please make at least 3379MB available in /var/lib/mongodb/journal or use --smallfiles