Rescue mode will allow you to perform maintenance on your server while it is not booting or connectivity was broken due to misconfigurations, hardware faults etc.
As a first step you will need to access server info and actions "My Services" >> Select your server from the list that you want to boot into rescue mode >> Select tab "Manage server".
To boot server into rescue mode, from "Server actions" menu press on button "Rescue mode".
Next in the "Server info" menu you will be able to see SSH credentials for the server where it was mounted. Use these to access and repair your server.