This is most often due to a blow on the nose, rupturing some of the tiny blood vessels, and can be quite common in children. Occasionally a nosebleed is sparked off by very hot, humid weather, or my excitement, and if they occur frequently, professional treatment should be sought.
Soak a pad of cotton wool in cold, or even iced water with a couple of drops of Lemon oil added, and place firmly across the bridge of the nose. A more effective method is to place a small piece of this as a plugin in the affected area.
Make an infusion of Yarrow Oil (Achillea millefolium). When cool, soak a piece of cotton wool or soft cloth in the infusion and place it on the soft part of the nose. Hold firmly in place.
The essential oil helps to speed up clotting. Sometimes placing a cold compress on the back of the neck can be helpful in reducing the local blood flow too.