It would appear that Broadsword Online Games is experiencing some issues with the login servers for Ultima Online:

We are having issues with our log in servers, if you log out of UO you will not be able to log back in. We expect the log in servers to be down for approx. 2 more hours, if something changes I will post and let you know. I will post another update if they are not back by 6:30 pm EST.

While the problem was, I gather, initially limited to the Enhanced Client, that changed a bit later on in the day:

9:35 pm EDT update: While in the process of fixing the Enhanced Log in servers we are having to bring down the Classics log in servers also. We are still working on this and I will update when I hear anything. Sorry guys!

And the problem is still ongoing as of the last update posted to the UO site:

Update: Players can log in with the Classic client but the Enchanced Client is still having log in issues. Will keep you updated

Hopefully Broadsword can get this resolved soon. And while I’m at it, hopefully it’s not the result of anything malicious.