Sunflowers do tend to droop when they get heavy with seed. You can cut off that heat or tie it up if there is a fence nearby or you can create a stalk. Wishing you the best.
Tie it to adjacent stalks with a strip of cloth or soft rope.
See if this linkk explains what is going on with your sunflowers:
Long bamboo pole or garden stake. I use old nylons to tie plants up without damaging stalks.