a bolt out of the blue晴天霹雳，意外事件

【例句】
The news hit me like a bolt out of the blue. I had thought I surely passed the entrance examination.
这消息使我如一遭晴天霹雳，我原以为我一定会考上大学。

The whole thing was a bolt out of the blue.
这件事，整个来说，简直是晴天霹雳。