Missing You by Meg Cabot
Jess is a shadow of her former self, her powers gone, Lightning Girl no more. But Jess knows her powers aren't the only thing that's gone: she's lost herself, as well. Starting over in a new town, intent on finding a new life in college, Jess is less than happy when Rob Wilkins, her ex, shows up at her door, begging her to help him find his long lost sister. But how can Jess, her powers gone, find anyone? Let alone the sister of a man she once loved (and believed, wrongly, loved her back) when she can't even find herself?