I was running a script on a server that took a while to complete. Since I did not want to lose any output I ran it inside a screen session. Of course, during the course of this, my SSH connection dropped and when I logged in again, and tried to reattached to the session I got this:

[root@ultra ~]# screen -list
There is a screen on:
        21884.pts-0.ultra   (Attached)
1 Socket in /var/run/screen/S-root.
[root@ultra ~]# screen -r 21884.pts-0.ultra
There is a screen on:
        21884.pts-0.ultra   (Attached)
There is no screen to be resumed matching 21884.pts-0.ultra.

As it turns out, you have I had to first detach and logout first, to be able to attach to the session:

[root@ultra ~]# screen -D -r  21884.pts-0.ultra

Useful to know…