I upgraded a system today from Ubuntu 9.10 to 10.04, everything was fine until I rebooted after the upgrade, since then I've been receiving SSH Connection Refused messages from putty. I presume a firewall or IPtable setting has been edited / set during the upgrade, but I'm unsure as to how to fix this issue as we're currently locked out.
If you're locked out, with no physical access, there's little you can do.
Can you remotely reboot it? Might be worth a try.
Are you using keys? The permissions may have changed, try using passwords if you can.
Can you use a different user?
All in all, if you have no access to the server you can't fix it. Get someone who does have physical access to go and have a look.