First, if a person forgets a Sunnah (supererogatory act of worship following the example of the Prophet) of Salah, there is no harm on them and their Salah is valid.Second, Qiyam-ul-Layl is a stressed Sunnah. The Prophet (peace be upon him) never left it, whether he was at home or traveling, whether he was sick or healthy. However, if a person misses Qiyam-ul-Layl because of sleep, sickness or forgetfulness, it is permissible to make up for the Witr (Prayer with an odd number of units) the next morning by performing two Rak`ahs (units of Prayer) at the time between sunrise and noon. It is related from
`Aishah (may Allah be pleased with her) that
"When the Messenger of Allah (peace be upon him) missed the Witr due to sleep or pain, he used to offer twelve Rak`ahs during the daytime."
Muslim in his Sahih (authentic) Book of Hadith) He (peace be upon him) usually offered eleven Rak`ahs in pairs, and then offered one Rak`ah as Witr. (Part No. 6; Page No. 107) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.