Is the contraction mightn't a verb or pronoun?

The contraction mightn't is a shortened form of the auxiliary verb 'might' and the adverb 'not'.

'Might not' is most commonly used to express negative possibility:

• She might not have taken the bus.
  
• She mightn't have taken the bus.

A pronoun is a Word that takes the place of a noun in a sentence. The pronoun 'she' takes the place of a noun (name) for a female as the subject of the example sentence above.