The most common access method for running instances is probably ssh, but this requires you have setup your instance with ssh keys and you have arranged for it to be running ssh with a public ip and opened the ssh port in your security group configuration. If you haven't done this or you are trying to debug a problem image OpenStack can be configured to provide a VNC console, be aware that VNC is an unencrypted protocol so you should be cautious what you type across that link. See the Getting Started With VNC Proxy section for details on how to configure and connect to this service.
