This solution is for when you have accidentally dragged the chat bar off screen completely, not when you have turned it off via preferences. So I thought I would post the solution since I have now had this happen to me twice in the last 6 months.
Enter full screen mode > Remove all toolbars > Then hover your mouse over the bottom left corner of the screen > When you see the arrows appear click and drag your mouse right to bring the Nearby Chat bar back out on to the screen
I had my Nearby Chat bar missing for about a month on Firestorm before I found the solution to bring it back. I hope this helps other SL players.