Rashi on Numbers 6:11/רש"י על במדבר ו׳:י״א

1מאשר חטא על הנפש [AND THE PRIEST … MAKE EXPIATION FOR HIM] FOR THAT HE HATH SINNED BY THE DEAD — i.e., that he has not been on his guard against defilement by a corpse. — R. Eleazer ha-Kappar said, “his sin consists in that he has afflicted himself by abstaining from the enjoyment of wine (Sifrei Bamidbar 30; Nazir 19a).

2וקדש את ראשו AND HE SHALL HALLOW HIS HEAD [THAT SAME DAY] by beginning again the counting of his Nazaritehood (i.e., he resumes his Nazaritehood).