This will depend on your Auto Replace setting. If it is set to "True" then the proxy server will assign you to a new IP address that is closely related to the IP that went offline. If it is set to "False" then the proxy server will not assign you to a new IP address. If it is set to "Any" then the proxy server will assign you to a random new IP.
You can set the Auto Replace settings from within your dashboard or through the Proxyrack API.