The electricity cut out during Jumu’ah prayer and they were not able to continue following the imam
Question

                We were praying Jumu’ah, and our mosque is on several levels. During the first rak’ah the electricity cut out and the worshippers who were praying on the second and third levels were not able to continue following the imam, so they prayed Zuhr. Is what they did correct?.
Praise be to Allah.
    If that happened before rukoo’ (bowing) in the first rak’ah, 
    then what they did was correct. But if that happened after bowing with the 
    imam then they should have completed it as Jumu’ah, with two rak’ahs. 

    Shaykh ‘Abd al-‘Azeez ibn Baaz Aal al-Shaykh was asked about 
    a similar situation and he said: Whoever catches up with one rak’ah of 
    Jumu’ah then is faced with a problem such as the electricity cutting out and 
    the like, should complete it as Jumu’ah on his own, i.e., he should pray the 
    second rak’ah then say the salaam, because in al-Saheehayn it is 
    narrated from Abu Hurayrah (may Allaah be pleased with him) that the 
    Messenger of Allaah  (peace and blessings of Allaah be upon him) said: 
    “Whoever catches up with one rak’ah of the prayer has caught up with the 
    prayer.” One catches up with the rak’ah by catching up with the rukoo’ 
    (bowing). The one who does not catch up with rukoo’ with the imam has not 
    caught up with the rak’ah. Based on this, if the people mentioned in the 
    question did not catch up with anything but the opening takbeer with the 
    imam, then they should have prayed it as Zuhr. End quote. 

    Majallat al-Buhooth al-Islamiyyah 
    (61/83).