Fledglings are humans with vampire blood in their system; they are in transition to Turning into vampires but are considered not-yet or newly born.
To be truly born, they must die, be buried, and successfully dig themselves out of their grave. Upon being reborn, they are immediately in a frenzy for blood and, nearly feral and starving, will be unable to control themselves and will kill anything or anyone close to them just to drink blood.
If they are unable to make their way out of their grave, or if no one is there to help them, they stay trapped like rats under the earth, starving underground. However, if they were not buried to begin with, they will stay in the state of transition, dead but not quite dead, their soul trapped in the lifeless corpse; never waking and moving on.