Try to little by little remove her anxiety. If you're not sure you can, get help from an expert. Was it due to an accident? First have her think about the many times she rode a horse. How she would handle it smoothly. Then have her think about starting riding again. What is it that frightens her? Tell her to imagine that she's doing all the riding process with no anxiety (NOT trying not be anxious). Then in the real world, have her ride a horse slowly while someone's standing beside the horse and is watching after her. Little by little, anxiety will go away. Also be careful not to make a fuss out of all these, not to suggest her that should be afraid of it. Hope you make it!
Slow exposure and encouragement. I love horses but I can understand how someone could be afraid of them. You have to take it step by step and encourage her to re-embrace that part of her life. But do it slowly so that she can regain the confidence properly.
First you need to understand the reason what made her lose her confidence. Then, the best thing you can do is be an example to her. Show her how safe and fun it is to ride a horse and how much she is missing on it.