Does an Imaam's death invalidate the prayer?
14-1-2014 | IslamWeb
Question:
If the Imaam passed away during the congregational prayer and I am praying behind him, what should I do?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If something happened to the Imaam (i.e. the one who leads prayer) during the congregational prayer preventing him from completing the prayer such as passing wind, remembering a ritual impurity, death or the like, the prayer of the congregation is valid according to the opinion of the majority of Muslim scholars. In such a case, the worshippers can choose someone from them to replace the Imaam and complete the congregational prayer.
The Maaliki scholars believed that choosing a successor of the Imaam (to complete the congregational prayer) in the Friday prayer is obligatory, since the congregational prayer is a due condition of the Friday prayer. As for other congregational prayers, the worshippers have three choices: 1) either to choose someone from among themselves to replace the Imaam and complete the congregational prayer, 2) complete their prayers individually or 3) split up and some complete the congregational prayer while others pray individually. All the previous choices are allowable according to them.
The evidence of the majority of Muslim scholars is what Al-Bukhari 
cited about the story of the killing of ‘Umar 
and how ‘Abdur-Rahmaan ibn ‘Awf was chosen as a successor of the Imaam to complete the congregational prayer after ‘Umar was stabbed.
Hence, it becomes clear for the questioner that if something happens to the Imaam during the congregational prayer preventing him from completing it such as death or the like, the prayer of the worshippers is valid and the death of the Imaam does not invalidate their prayer.