Hi Monika,
Welcome to these National Gardening Association forums.
"...does anyone know what would cause a soon to be blooming zinnia to wilt over?"
I don't understand what you are asking. You say it is "a soon to be blooming zinnia" and then apparently you say it has lots of other stems that produced beautiful flowers. If it is a single plant already with several blooms and just one branch is wilted, why not just cut off the wilted branch?
A zinnia
plant could wilt for lack of water. There are lots of reasons why a zinnia branch could "wilt". It could be just mechanical damage from a bird landing on the branch and being too heavy for it. A dog could have bumped into the plant. Some more description could help explain your problem, or possibly a picture of it would help.
ZM
