原因：MongoDB未正常关闭导致的

解决：

1. 找到dbpath里的mongod.lock文件，删除mongod.lock文件；例如（/usr/local/mongodb/data文件中）
2. 以管理身份启动：sudo /usr/local/mongodb/bin/mongod --config /usr/local/mongodb/bin/mongodb.conf

   [ec2-user@ip-172-31-43-102 bin]$ ./mongod --config /usr/local/mongodb/bin/mongodb.conf
   about to fork child process, waiting until server is ready for connections.
   forked process: 4099
   ERROR: child process failed, exited with error number 14
   To see additional information in this output, start without the "--fork" option.
   [ec2-user@ip-172-31-43-102 bin]$ sudo su
   [root@ip-172-31-43-102 bin]# cd /usr/local/mongodb/bin
   [root@ip-172-31-43-102 bin]# ./mongod --config /usr/local/mongodb/bin/mongodb.conf
   about to fork child process, waiting until server is ready for connections.
   forked process: 4131
   child process started successfully, parent exiting

3. 查看进程是否运行 ps -aux | grep mongod