Did the cold temperature of the lunar south pole cause Chandrayaan-2's on board electronics to fail?
tl;dr: Probably not, the spacecraft was likely in sunshine at the time, and would have also been built and insulated to withstand regular short eclipses.
Currently we don't know what happened, or if the mission is lost.
At the time of the second communications loss (the first loss of communications with the Deep Space Network was recovered per Wikipedia) the spacecraft was close to landing at 70.9°S 22.8°E which certainly would be sunlit for the landing; the Moon had already reached first quarter so this longitude will be warming up nicely.
Since the loss of contact was close, the spacecraft is likely to have been sunlit as well, and for several tens of minutes beforehand.
But I think the important point is that the spacecraft was built to work in a polar lunar orbit which regularly goes into the shadow of the Moon. So the chances that the electronics got too cold in orbit or during descent are very small.
More likely something other than "cold electronics" went wrong.