What you can do is you can start the MongoDB server by executing the following syntax as root:
>sudo service mongod start
Or just shut down your server via the client. And then start it.
>mongo --host "hostname where server or IP" --port "port of mondodb server" --username "username" --password "password"
>use admin
>db.shutdownServer()
Want to learn more about MongoDB? Go through this MongoDB course for more insights.