Misfortune
Calamity, Disaster
noun
A disaster that is difficult is particularly difficult, making it more of a misfortune. Obviously a disaster that wasn't difficult wouldn't really be that bad, so a difficult one gets the "misfortune" or calamity label.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
それはとんだ災難でしたね。
That was quite misfortunate, wasn't it?
さっきは災難だったね。あの先生、みんなにうるさいからさ、気にしない方がいいよ。
That was too bad earlier. That teacher is always nagging everyone, so it's best not to worry about it.
せっかくの春休みなのに、インフルになるとは。災難すぎる。
To think I'd come down with the flu during my precious spring break. What a disaster.