What should you do if you did not pray witr because you slept through the appointed time or because you forgot?
Pray it as you would normally pray it. The evidence for this is the ḥadīth of Abū Sa‘īd Al-Khudrī in which he said: Allāh’s Messenger (May Allāh praise him and Peace be upon him) said, “If somebody did not pray witr because he slept through the appointed time or because he forgot, he should pray it when he remembers.”
[This ḥadīth was graded as ṣaḥīḥ by Al-‘Allāmah Aḥmad Shākir in his checking of Al-Tirmidhī]
Pray it during daytime as twelve rak‘ah (units of prayer). This is what was narrated by ‘Ā`ishah (may Allāh be pleased with her) from the actions of Allāh’s Messenger (May Allāh praise him and Peace be upon him) when she said, “When he did not pray the night prayer (qiyām al-layl) because he was overcome by sleep or pain, he would pray twelve rak‘ah during daytime.”
[Reported by Al-Imām Muslim]