在ubuntu中尝试此mongo命令时，出现此错误。

    ritzysystem@ritzysystem-Satellite-L55-A:~$ mongo
    MongoDB shell version: 2.6.1
    connecting to: test
    2014-10-06T12:59:35.802+0530 warning: Failed to connect to 127.0.0.1:27017, reason: errno:111 Connection refused
    2014-10-06T12:59:35.802+0530 Error: couldn't connect to server 127.0.0.1:27017 (127.0.0.1), connection attempt failed at src/mongo/shell/mongo.js:146
    exception: connect failed

我该如何纠正这个问题呢？