This method only works if you want to repair your data files without preserving the original files
To find where you dbpath resides- vim /etc/mongodb.conf
check for option dbpath=
(I have dbpath=/var/lib/mongodb)
Default: /data/db/
Typical locations include: /srv/mongodb, /var/lib/mongodb or /opt/mongodb .
Replace the /var/lib/mongodb with your dbpath
sudo rm /var/lib/mongodb/mongod.lock
sudo mongod --dbpath /var/lib/mongodb/ --repair
sudo mongod --dbpath /var/lib/mongodb/ --journal
(Make sure that you leave you terminal running in which you have run above lines, dont press 'Ctrl+c' or quit it.) Type the command to start mongo now in another window.
Hope this works for you ! for those who want to repair your data files while preserving the original files mongo recoversudo rm /var/lib/mongodb/mongod.lock