If the namaz is underway in jamat in a masjid at its different floors. A sound system is used in masjid to connect with muqtadeen at different floors. Due to some reasons sound system stops working. Muqtadeen in the basement or at any floor of masjid are completely disconnected as there was no takbeer being called. Under the given conditions, after realizing that the connection with imams is completely lost, can anyone in the front row of basement lead the jamat to complete the namaz or each muqtadi will have to complete namaz individually.
بسم الله الرحمن الرحيم
When the link with the imam was discontinued completely then now all people will break the niyat and offer salah individually or someone else will become imam and they shall complete their salah. It is not correct for any other person to become imam to complete the remaining salah, as salah will not be valid in this case.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.