What is the ruling on someone who entered into a state of ihram for Hajj while residing in Mecca, then returned to his dwelling and took off his ihram before midnight due to losing his companions and his ignorance of the rituals, and did not complete his Hajj? And what is incumbent upon him?

Whoever has entered into the state of ihram for Umrah or Hajj must complete their rites, based on the verse: (And complete the Hajj and Umrah for Allah). The loss of companions is not an excuse for abandoning completion. You must repent for your negligence in learning religious rulings.

You must complete the pillars you omitted, namely Tawaf al-Ifadah and Sai of Hajj, because they are not waived. You are liable for a sacrifice (dam) for the duties you omitted, such as throwing the pebbles (rami al-jamarat), spending the night in Mina (al-mabit bi Mina), and offering the sacrifice (al-nahr) (if you were performing Qiran Hajj), as well as shaving/cutting the hair (al-halq) according to the majority of scholars.

As for prohibitions, such as sexual intercourse before completing the rites, there is nothing upon you if it was due to ignorance of the ruling. In summary: You must repent, complete Tawaf al-Ifadah and Sai, and offer three sacrifices (dams) for the omitted duties (shaving/cutting, throwing pebbles, and spending the night in Mina), and a fourth for the sacrificial animal (hady) if you were performing Qiran Hajj.