ubuntu mongodb 出现 exception: connect failed

被这个问题搞崩溃,百度google都试了 依然不行,重装了也不行!!
找了各种方法都不行,高手可用指点一下吗?

错误如下:

2017-03-21T19:16:41.552+0800 W NETWORK  [thread1] Failed to connect to 127.0.0.1:27017, in(checking socket for error after poll), reason: Connection refused
2017-03-21T19:16:41.552+0800 E QUERY    [thread1] Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed :
connect@src/mongo/shell/mongo.js:237:13
@(connect):1:6
exception: connect failed

真心希望懂的人来帮忙一下

楼下大哥补充一下~

我启动mongodb会出现这样

Failed to start mongod.service: Unit mongod.service not found.

监听端口是这样的

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:3306            0.0.0.0:*               LISTEN      953/mysqld
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      973/apache2
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      888/sshd
udp        0      0 139.196.187.137:123     0.0.0.0:*                           961/ntpd
udp        0      0 10.25.105.32:123        0.0.0.0:*                           961/ntpd
udp        0      0 127.0.0.1:123           0.0.0.0:*                           961/ntpd
udp        0      0 0.0.0.0:123             0.0.0.0:*                           961/ntpd
udp6       0      0 :::123                  :::*                                961/ntpd

确定是没有开启!,但是问题是启动不了~~~~

根据楼下大哥提示:

systemctl start mongod.service

会出现:

Failed to start mongod.service: Unit mongod.service not found.

但是我mongodb确实是安装好了~