I absolutely bleached my black-and-white skirt trying to make the white whiter how do I brighten the black now? And can it be saved

Sorry, but No. Bleach lifts the color out completely, that is why labels will say "do not bleach", it is irreversible. The black is not just hiding, it is no longer there. The only way to get the black back would be to somehow use fabric dye and paint it on.