First of all, it is highly recommended to perform Witr, as it was practiced by the Prophet (peace and blessings be upon him) and he encouraged Muslims to practice it. It is permissible according to the majority of Muslim scholars to make up for a missed Witr Prayer. However, there is a difference of opinion over what time it may be made up.
The prominent Muslim scholar, late Sheikh Sayyed Sabiq, the well known Azharite scholar and the author of “Fiqh As-Sunnah” states the following: The majority of scholars maintain that it is permissible to make up for a missed Witr prayer. Abu Hurairah (may Allah be pleased with him) reports that the Prophet (peace and blessings be upon him) said: “If the morning approaches, and you have yet to pray Witr, you should pray the Witr Prayer.” Abu Dawud reports on the authority of Abu Sa`id Al-Khudri that the Prophet (peace and blessings be upon him) said: “If one of you sleeps (and thus misses) the Witr Prayer or forgets it, he should pray it when he remembers it.”
Generally speaking, there is a difference of opinion over what time it may be made up. The Hanafi jurists hold the view that it should be performed during those times in which it is not forbidden to perform prayer. The Shafi`is, however, maintain that it may be made up during any time of the night or day, while, according to, both Imam Malik and Imam Ahmad, a missed Witr Prayer is to be made up for after Fajr.