Hi Monika,
Your pictures show the wilted branch very well. Since only one branch is affected, and the rest of your zinnias seem too be fine, the only concern is whether this is a problem that is likely to spread. The pair of big leaves next to the dying or dead part of the branch seem fine. If a kid shot the zinnia with a BB gun, some damage like this could occur.
I have quite a few zinnias, and the only time I have seen damage like this was from a stalk borer - a moth larva caterpillar that burrows into the stem and eats it way internally inside the stem, which dies. But this doesn't look like typical stalk borer damage -- they usually enter the central stem via a hole near ground level.
As long as this problem doesn't start to spread, you can essentially do nothing and watch for further developments. I'm only half kidding about the BB gun. Keep us posted for further developments. Your zinnias are a little crowded, but they seem to be healthy looking, except for that one dead branch.
ZM
