Hi Peter,
Try hanging out with her as often as you can one on one. Start with suggesting activities to her that you know she would enjoy.
If you want to build sexual attraction it would be best to pick activities that require physical proximity and a lot of bodily contact.
These could be activities such as dancing, theme parks, etc. Again, it really depends on what she enjoys.
Consistently experiencing the kind of physical proximity and bodily contact that these kind of activities tend to require may eventually get her back to a place where she starts feeling a stronger sense of attraction towards you like she once did.
As long as you are patient in engaging in these activities and avoid trying to force the attraction, you may ultimately get out of the friendzone with her.