If some of the members of the congregation are praying outside the mosque and the power cuts out
Question

                I came to the mosque late for ‘Isha’ prayer, and there was no room inside the mosque, so we prayed outside the mosque. During the last rak’ah the power cut out, and the sound of the microphone was cut off from us and we could not hear the imam. What should we do in this situation?.
Praise be to Allah.If some of 
    the members of the congregation were outside the mosque, and they were 
    unable to follow the imam because the power cut out, then what is prescribed 
    in this situation is for one of the worshippers who are near the door of the 
    mosque to say the takbeer out loud so that those who are outside the mosque 
    can hear and can follow the imam in the prayer. If he does not do that then 
    they have the choice of either completing the prayer individually or sending 
    one of their number forward to lead them in completing the prayer in 
    congregation. This is better, so that the worshippers will not be confused 
    and frustrated as a result of the imam’s voice being cut off. 
Shaykh Ibn 
    Baaz (may Allaah have mercy on him) was asked: A group of people prayed 
    Jumu’ah in the basement of the mosque, and during the prayer the power cut 
    out, and they could no longer hear the imam’s voice. One of them went 
    forward and led them in the rest of the prayer. What is the ruling on their 
    prayer, knowing that he completed the prayer as if it were Jumu’ah? What 
    would be the ruling if no one went forward to lead them – should each person 
    complete his prayer on his own? If that is permissible, should they complete 
    it as Zuhr or as Jumu’ah, since they had listened to the khateeb and started 
    to pray with the imam and prayed one rak’ah with him? 
He replied: 
If the 
    situation is as the questioner has described, then the prayer of all of them 
    is valid, because whoever catches up with a rak’ah of Jumu’ah has caught up 
    with Jumu’ah, as is stated in a saheeh hadeeth from the Messenger of Allaah 
     (peace and blessings of Allaah be upon him). Even if no one went 
    forward to lead them and each person prayed the second rak’ah by himself, 
    that is sufficient, just as the one who joins the prayer late prays whatever 
    he catches up with with the imam, then he makes up the other rak’ah by 
    himself, because of the general meaning of the words of the Prophet  
    (peace and blessings of Allaah be upon him): “Whoever catches up with a 
    rak’ah of prayer has caught up with the prayer.” End quote from Majmoo’ 
    Fataawa Ibn Baaz (12/331). 
And Allaah 
    knows best.