"I looked at her."
Translation:Spojrzałam na nią.
"Nią" is not only Instrumental, but also a possible Accusative form which appears after a preposition. So if it's bare Accusative (I saw her = Widziałem ją) it's only 'ją', if it's some preposition that need Accusative, 'ją' changes into 'nią'.
Same applies for other genders. go/niego for masculine; je/nie for neuter and for not masculine-personal plural, ich/nich for masculine personal plural.