Croup is just a virus so there is no treatment for it, but you can be prescribed Predmix or Redipred for it, which is just a steroid that opens their airways. It could just have been a once off thing and tonight she will be fine, but sometimes they can be fine through the day and the cold air at night can bring it on again. It's up to you as to whether you think she needs the steroid or not but if she is happy enough and not distressed by it then she may be fine without it.