。
She felt something touch her neck.
Literal
She [topic-は] something [subject-が] neck [on-に] touch [object-の] felt.
~のを感じる ('to feel that...') uses の to nominalize an entire small event — 何かが首に触れる ('something touches her neck') — and present it as the perceived object. Without の, 感じる couldn't take the embedded clause directly. 触れる ('to touch, brush against') is gentler than 触る ('to touch deliberately'); it suggests light contact, which here makes the sentence faintly unsettling. The が on 何か focuses the unknown agent: it wasn't a known person.