Yeni: i didn't see you at kiki's party yesterday. Where were you? Ira: i didnt come. I would have come if i...... So busy

"Was not"

cause they are talking about thing that has passed. The sentence must be created in past form,,

3rd Conditional (past unreal)
I would have come if I HAD NOT BEEN so busy.

karena faktanya dalam past tense maka gunakan pengandaian tipe 3.