This is sort of a spin-off of the sedevacantist question. How should this situation be handled, in the case of a latae sententiae excommunication?
Can. 1331 §1. An excommunicated person is forbidden:
1/ to have any ministerial participation in celebrating the sacrifice of the Eucharist or any other ceremonies of worship whatsoever;
2/ to celebrate the sacraments or sacramentals and to receive the sacraments;
3/ to exercise any ecclesiastical offices, ministries, or functions whatsoever or to place acts of governance.
Can. 1364 §1. Without prejudice to the prescript of ⇒ can. 194, §1, n. 2, an apostate from the faith, a heretic, or a schismatic incurs a latae sententiae excommunication; in addition, a cleric can be punished with the penalties mentioned in ⇒ can. 1336, §1, nn. 1, 2, and 3.