Al-Qasim asked Sayyidah Aishah (may Allah be pleased with her) about a man who falls unconscious and misses the prayers of a day or two days or more than that. She said that the Messenger of Allah (peace and blessings be upon him) said: "There is no making up for him, except the prayer in which he regains consciousness; he will pray that."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1820