Recently, I experienced that Mongodb was not starting after a system crash. When I ran

service mongod restart

mongodb crashed with error in log file. The error said "exception in initAndListen: 12596 old lock file, terminating"

If you ever encounter this issue, the solution is to start the mongod instance using the –repair option by using the following command

sudo -u mongodb mongod --repair --dbpath /var/lib/mongodb/