Easiest thing when a thread has slipped off is just to adjust the date filter, as Joe said - the one that is level with Create a New Thread towards the top of the forum page.

When the Mudcat has been down for a bit, doing that is a good way of starting the conversations rolling again. In which case rather than entering any word into the panel, leave it blank, and just set the date thingy at three days or whatever, instead of the default which is one day.