Is hadn't a noun or pronoun?

The Word hadn't is a contraction, a shortened form of the verb "had" and the modifying adverb "not".
The contraction hadn't functions as a verb or auxiliary verb in a sentence.

Examples:
He had not eaten so I made him a sandwich.
OR
He hadn't eaten so I made him a sandwich.

A noun is a Word for a person, a place, or a thing.

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