They are renewed.
Generally, this involves very rapid decay of the physical form with spontaneous generation of living material (beatles, various colorful fungi, flowers, worms, what have you), accompanied by pyrotechnic immolation of the shell of negative energy that once housed thier soul. Generally the blast of radiance is strongest through windows on the soul - the eyes and mouth, and to a lesser extent the tips of the fingers.
In any event, the undead ceases to exist as an entity within a few seconds.