Try following steps.
sudo apt-get purge mongodb mongodb-clients mongodb-server mongodb-dev
sudo apt-get purge mongodb-10gen
sudo apt-get autoremove
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 7F0CEB10
echo deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen
sudo rm /var/lib/mongodb/mongod.lock
sudo apt-get update
sudo apt-get install mongodb-10gen
mongod --repair
sudo service mongodb start
mongo
Its working in my system.