Oh, wow. This brings me back. I remember that the cause was found, but I don't think there's any way to fix it without loading a save or restarting the chapter. I'll do some digging around and see what I can turn up.
Edit: Okay, here's what I've got. If you ask to go to the courtroom while you're in your cell and then finish the trial, you get stuck in the cell. If you get out of the cell (ask to talk to your lawyer), then enter the courtroom by talking to the baliff at the front doors, you'll be free when the trial ends.
There's a save file from right after the trial that you can use to progress. Just follow the directions in
this thread.