A few days ago I had a problem where mongodb crashed and I couldn’t start it up. I realized that this problem occurred because the disk was full. So I  rm -rf  a few things and try to sudo service mongod start . But this wasn’t successful. I googled this problem and I quickly found the solution:

sudo rm /var/lib/mongodb/mongod.lock
sudo mongod --repair
sudo service mongodb start

After this mongodb started without any problems.