From what I can see in your screen grab you have a single polygon face and it appears selected. I cannot see any reason why extrude would not work.
Try deleting that simple polygon, recreate it, and then see if you can extrude the new one. If you cannot post again and if possible attach a .ma save of your model so I can see if I can replicate your problem.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675