Mongodb failed to start?

  mongodb, question
 MongoDB shell version v3.4.4
 connecting to: mongodb://
 2017-05-14T23:17:25.098+0800 W NETWORK  [thread1] Failed to connect to after 5000ms milliseconds, giving up.
 2017-05-14T23:17:25.099+0800 E QUERY    [thread1] Error: couldn't connect to server, connection attempt failed :
 exception: connect failed

Mongo server is not started. Please enter mongo to start the server before Mongo can connect and follow up.