This is the 9469th most frequent Japanese word.
笑え
"笑え" means "laugh" or "smile" (imperative form).
In this sentence, "笑えなかった" is the negative past form, implying that "laughing" was not possible due to the joke's nature.
彼のジョークには誰も笑えなかった。
No one could laugh at his joke.
Here, "笑えました" is the past polite form, expressing the natural occurrence of laughter.
この写真を見て、思わず笑えました。
Seeing this picture, I couldn't help but laugh.
"笑えない" is the negative form, showing the difficulty of finding humor in a given scenario.