Rhubarb leafstalks (the part we consume) should not be harvested if the leaves are wilted and limp after a hard freeze. The leaves of the rhubarb plant, which should never be eaten, contain a toxic substance called oxalic acid. After a hard frost oxalic acid may move from the leaves into the leafstalk. Under normal harvest the leafstalk is cut at the base and the leaf blades are trimmed off. If consumed the oxalic acid can cause you to become sick. All rhubarb leaf stalks/petioles that have been exposed to freezing temperatures should be removed and discarded. The re-growth is safe to eat.
(University of Illinois Extension. May 1, 2014)