Experts and BHG readers answer.
Tomato plant leaves
There are a couple of things that can cause leaves on a tomato plant to curl up. One is a physiological condition called tomato leaf roll. This usually happens later in the season when the plant is under moisture stress. Leaf curl can be caused by feeding from aphids or other insects. Do you see any insects inside the curled leaves?
It's also possible that your tomato plant was exposed to broadleaf weed killer herbicide drift. Did you or your neighbors apply a weed control product to the lawn recently? Tomatoes are quite sensitive to 2,4-D, a common ingredient in lawn weed control products. Just a slight amount drifting through the air can cause tomato leaves to curl up.