While performing regular maintenance on a remote server it may happen that it does not respond anymore to remote commands. When this happens, a quick try would be to do a remote reboot of the server.
As a first step you will need to access server info and actions "My Services" >> "Dedicated servers" >> Select your server from the list that you want to reboot >> Select tab "Manage server".
To boot server into rescue mode, from "Server actions" menu press on button "Reboot".
Now allow the server 60-120 seconds to fully reboot and have SSH service started again.