In English we use possessive adjectives in front of body parts (my, your, his, etc.), but in French a definite article is typically used when the owner of that body part is obvious (le, la, l'). J'ai une douleur dans le cou. Clearly I have a pain in my own neck and not someone else's so the definite article "le" is most appropriate here. You will typically find this construct with the verb avoir.

Here is a link with more info: https://french.kwiziq.com/revision/grammar/using-le-la-les-with-body-parts-and-clothing-definite-articles

French requires an article in front of nouns most of the time. Here "une douleur" translates simply to "pain" in English.

Pain or douleur is a feminine noun in French, and so it requires an article. In French this is a specific, singular pain in the neck. To describe more general pain, you can use "la douleur."

I couldn't find any reference materials, but here is a good example of "douleur" being used in real life.

In French, you do not use the personal possessive adjectives (mon/ma/ton/ta/etc.) when the owner of the body part is obvious. Since you can't have a pain in someone else's neck, you use the "le."

J'ai mal à la tête. → My head hurts.

In French when the owner of the body part is evident, the personal possessive adjective is often dropped in lieu of a definite article "le, la, les."

Salut !

Sorry for the delayed response. J'ai mal au cou and j'ai mal dans le cou for "my neck hurts" are additional ways to express the same idea. From my experience, it is more common to use à la/au + body part rather than dans la/le + body part, but both are valid.