A damper can malefaction for several reasons, sometimes it just operator error.
If your damper is broken there are several repair available and after market parts to help you out. A replacement handle is available for most internal dampers while newer more efficient top mount dampers can be installed. A top mount damper installs on the top of the chimney with a cable and handle running down to the firebox. When closed the damper seals off at the top preventing rain, animals from getting into the chimney. Most importantly though it prevents air loss and keeps your warm air in the house and that cold outside air, well outside!
The Chimney Clinic