Words fail (someone) is an idiomatic expression of inability to describe something with words, (usually) being overwhelmed with some emotion (shock, surprise, fear). There's also another expression with a similar sense: (being) lost for words.
Here are examples from three different books:
My heart is so full, yet my words fail me.
Sometimes, however, words fail us as speakers and we find a
picture truly is worth a thousand words.
I now realize most people scatter in times of true suffering,
overwhelmed and awkward when words fail them as they attempt to
There's also a less idiomatic version: words fail (e. g. to describe something).