noun

an event causing great and often sudden damage or distress; a disaster.

emergency measures may be necessary in order to avert a calamity

disaster,catastrophe,tragedy,cataclysm,devastating blow,crisis,adversity,blight,tribulation,woe,affliction,evil,misfortune,misadventure,accident,stroke of bad luck,reverse of fortune,setback,mischance,mishap,car crash,bale,mishanter