All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, for more benefit on the evidence for Hijaab and its conditions, please refer to Fataawa 81554, 83765, 83702, 81494, 354259 and 83033.
There is no disagreement among the Muslim scholars that the neck is part of the ‘Awrah, so a woman must cover it.
However, if there is any pressing need that is valid in the Sharee'ah such as when covering it may cause unbearable harm – which is very rare – and she cannot wear any kind of clothes like silk, etc., then, in this case, it is permissible for her to uncover her neck only for the extent of dire necessity.
Know that uncovering it in front of non-Mahram men should be restricted only to the limits of dire necessity.
Allah knows best.