If you haven't injured your nose in anyway and your nose just suddenly started bleeding without a known reason then it could be caused by dry air that causes the blood vessels in your nose to crack and bleed suddenly or could be caused by inflammation in the nose or even deformities inside your nose.
Usually there's nothing serious going on if a nose suddenly bleeds and you're able to get it to stop after awhile.
However in some cases a nose bleed can be caused by an intranasal tumor but that's rare.
If your nose stops bleeding then it's usually nothing serious but if you're unable to get it to stop in a normal amount of time which is 15 to 20 minutes then you might need to see a doctor.