Just do those two commands for temporary solution:
sudo rm -rf /tmp/mongodb-27017.sock
sudo service mongod start
For details:
That shall be fault due to user permissions in .sock file, You may have to change the owner to monogdb user.
sudo chown -R mongodb:mongodb /var/lib/mongodb
sudo chown mongodb:mongodb /tmp/mongodb-27017.sock
For more details visit Documentation for installation