Is one allowed to take a haircut Erev Pesach after chatzos?
This is not permitted if the barber is a Jew, but it is permitted if the barber is a non-Jew.
Where one forgot to take a haircut earlier, and no non-Jewish barber is available, it is permitted to give oneself a haircut.
The basic halachah is ruled by the Mishnah Berurah 468:5, who permits a haircut from a non-Jew, and not from a Jew.
Yet, see She’arim HaMitzuyanim B’Halacha 113:6, based on the Shach Yoreh De’ah 399:12, who permits a person to give himself a haircut where no non-Jew is available. The reasoning for permitting this is in order that a person should not enter Yom Tov in an unkempt or ungroomed manner.
See, however Shemiras Shabbos Kehilchasa Ch. 42 note 140, who asks why the Mishnah Berurah does not make this point, thereby implying that the Mishnah Berurah maintains that it is not permitted.