Exsanguination
If a person is losing blood very quickly (for example, from a broken aortic aneurysm), they will only be conscious for 20 seconds to a few minutes.  If blood loss is slower, and the person does not get medical treatment, they may be conscious for a few hours.  However, they will get more and more confused and tired as they lose more blood.  Eventually the person will go into a coma because there is not enough blood left in their body to bring oxygen to their brain.  Finally their heart will stop and they will die.