Draft from fireplace
Since you say it's not working, I assume you mean disused fireplace.

Two insulation approaches (which don't depend on there being an operable damper, though closing a damper if you can figure out how will certainly help.)


Stuff rockwool or fiberglass batt insulation up into the throat of the fireplace (above the firebox) - also feel for drafts down low in case there's an outside air inlet, and stuff that if there is.
Cut one or more pieces of foam board insulation to roughly fit in the throat of the chimney, then use spray foam insulation to seal the edges tightly to the masonry. 


Neither of these need (nor should) be visible when looking at the fireplace normally. 

You can also fill the fireplace front opening itself, but that is more of a visual impact on the room with the fireplace in it and may be unacceptable as a result.

If you have not used it for at least 18 years and don't intend to, you might want to consider having the chimney externally capped off, as well, or even taken down, but those are more extensive projects rather than quick DIY solutions.