What should I pray after realizing the invalidity of my ablution during Fajr prayer: Do I pray only Subh, or do I pray both Fajr and Subh?

Perhaps the questioner means by "Fajr prayer" the two rak'ahs that precede the obligatory prayer. These are a Sunnah (supererogatory) and not Fard (obligatory). The questioner is only obligated to perform one prayer (the obligatory Fajr prayer), and it is legislated for her to pray the Fajr Sunnah before it.