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

or 

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