Is happened a pronoun

No, the verb 'happened' is the past tense of the verb to happen.

The noun form of the verb to happen is the gerund, happening.

Example: I don't know what happened to my notebook.

A pronoun is a Word that takes the place of a noun in a sentence.

Example: What happened to my notebook? I thought I left it here. (the pronoun 'it' takes the place of the noun 'notebook' in the second sentence)