What's the condition of the soil/potting mix? Dry, moist or soggy?
To know for certain, stick a trowel in it and look/feel.

It just looks dry. Stick your finger in the growing mix to determine if so.
If adequately moist, perhaps something damaged the stem?

there is something called nematodes (bad not good ones) that can get quite out of hand, try sugar around the plant next time, I had marigolds strange but true doing the same and cucumber plant beside it wilting during the day, poured sugar around them (fair amount) and within a couple of days they stopped wilting, nematodes choke on sugar