I found this particular post when I needed a solution for the exact same problem. I think I stumbled upon the answer by accident after reading through this and trying things in the "dead" screen window.
The situation: I use "set -o vi" in bash for my command line editor. I used "ESC k" to scroll back. I meant to press CTRL-C to cancel back to an empty prompt, but pressed CTRL-D by mistake. When I did that, that window in screen locked up...screen itself was still fine, and all other screen windows were fine.
The solution(?): I found that sending XOFF (ctrl-s) followed by XON (ctrl-q) it "unfroze" the window. Sending XON alone--without first sending XOFF--didn't work.
However, I've been totally unable to reproduce the CTRL-D lockup. I have no idea what set of conditions existed to cause it! Since I can't reproduce the problem, I can't test my solution, so I don't know if I actually found the solution or if it just happened to "fix itself" somehow after some length of time. *shrug*
If anyone gets the opportunity to test this, please reply! :o)