Bricks absorb water,a chimney needs an air flow through it to let it dry out,if the fireplace has been boarded up it should have an opening in it to allow air in at the bottom through the fireplace,the chimney pot should have a capped top to prevent rain coming in but allow air to circulate out of it.If you have closed it of recently this could be the problem,if it was boarded up years ago look for another cause.If it has been sealed off totally you will get this problem,open it up and leave it to dry out,it may take a few days in this weather,if it dries up vent it top and bottom.DON'T block it off.
Rich
July 2007