无法连接到服务器127.0.0.1:27017连接尝试失败MongoDB

我正在使用Ubuntu OS 16.04.我正在使用以下命令启动mongodb :: sudo service mongod start然后 mongo

I am working on Ubuntu OS 16.04. I am starting mongodb using commands:: sudo service mongod start and then mongo

它为我生成了此错误::

It generated this error for me::

MongoDB shell version v4.0.1
connecting to: mongodb://127.0.0.1:27017
2018-09-27T16:50:41.345+0530 E QUERY    [js] Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed: SocketException: Error connecting to 127.0.0.1:27017 :: caused by :: Connection refused :
connect@src/mongo/shell/mongo.js:257:13
@(connect):1:6
exception: connect failed

现在我无法启动mongodb.如何解决此错误以及这些错误的原因是什么. 我针对此问题尝试了所有解决方案，但没有找到任何解决方案.

Now I am not able to start mongodb. How to resolve this error and what are the causes of these error. I tried all of solution for this problem but didn't find any solution.