You can boot your Linode using a Finnix recovery profile to get access to your system for maintenance. Then, mount your existing root filesystem and reset a basic passwd file to cover at least root login. (Or if you have a backup somewhere, you can restore the file when running under Finnix).
The Rescue tab in the Linode manager will set up a dynamic Finnix boot, or you can define an explicit configuration using the Finnix initrd and kernel. I find the latter makes it easier to maintain my desired disk image mapping for the recovery boot, but if you don't have one already, using the Rescue tab is definitely quicker/easier.
See also
http://library.linode.com/troubleshooti ... escue-mode for some examples of the process.
-- David