I reset my computer but how can iTunes recognize my iPad content without erasing it

My computer was reset to factory settings but now my iPad is not recognized and considers my computer to be new, a different computer. When I attempted to sync the iPad the notification to erase my iPad content comes up in order to sync to the iTunes library. My iTunes library is incomplete and has no Apps, and 1/2 of my music loaded into it. HOW can I download my iPad content into theiTunes library without losing any data?