Correct spelling, explanation: happened is the correct form, because contrary to such words as planned or beginning, the n in happened is not a part of the stressed syllable. Usually, we double the last consonant when we add a suffix to a word ending with the combination of consonant-vowel-consonant. Here, however, the accent is not on the syllable including n, therefore happenned is incorrect, because in such case we usually do not elongate this sound.

Definition of happened:
verb, past simple and past participle of happen, to occur
Don’t worry about your mistake. Things like this happened to me as well when I was learning the profession.
This trip is the best thing that has ever happened to me.

Incorrect spelling, explanation: this form is incorrect because, in case of happened, the accent is on the first syllable, not the one with n in it. Otherwise, it would be spelled with double n, as planned. Despite the consonant-vowel-consonant rule, which says that we double the last consonant when adding a suffix, here stressed syllable is crucial because if there is no stress, we don’t elongate the sound, hence single n. Therefore happenned is incorrect, and the correct spelling is happened.

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling