Parashas Naso · Passuk 6:12
and they shall rededicate to GOD their term as nazirite, bringing a lamb in its first year as a penalty offering. The previous period shall be void, since the consecrated hair was defiled.
Chabad.org — English translation + Rashi
Source: Nazir 18a.
English: “And he shall separate to Hashem the days of his naziriteship” — \ldots what does this teach?
In context: After impurity the nazir must restart the count; the verse frames the renewed period as a fresh dedication.
Custom
Chazal detail the restart of the nazirite count after defilement by the dead.