check with either:
ps -edaf | grep mongo | grep -v grep # "ps" flags may differ on your OS
or
/etc/init.d/mongodb status # for MongoDB version < 2.6
/etc/init.d/mongod status # for MongoDB version >= 2.6
or
service mongodb status # for MongoDB version < 2.6
service mongod status # for MongoDB version >= 2.6
to see if mongod is running (you need to be root to do this, or prefix everything with sudo). Please note that the ‘grep’ command will always also show up as a separate process.
Check the log file /var/log/mongo/mongo.log to see if there are any problems reported.