Get a detached screen session

First you list all screen sessions:

screen -ls

would give following output:

$ screen -ls
There is a screen on:
    1973.pts-0.Debian-60-squeeze-64-minimal (09/04/2013 12:15:36 PM)    (Detached)
1 Socket in /var/run/screen/

then you select the specific session with its session pid and regain control of it

screen -r -D 1973

It is enough to provide only the PID (1973) of the screen process.