MongoDB service could not start or if you may get any errors, you might read to mongo log file using tail or less command.

 #  less /var/log/mongo.log


Read current status,
# tail -f /var/log/mongo.log

would summarize solution with below information,

If you see DNS-related errors there then check if you use correct DNS names which your system can resolve. 

If you have faced  network-related errors (such as could not connect to server) then check:

Check you use correct IP address and port number

Check your network links are up 

Check firewalls rules both on peer and your side are not blocking connection