Many friends are beginning to post photos of damage to plants, like these Aucuba in my garden, from this weekend's record cold. The real problem for most plants was not the cold itself, but the extreme and rapid drop in temperature. At my house, it dropped from 53 degrees at 3 p.m. to 0 degrees at 3 a.m. When this happens, plants--and especially broadleaf evergreens-- don't have time to react. Normally, they would draw more water down into their roots, increase the concentration of sugars in their leaves (antifreeze for evergreens), or both. This helps to keep ice crystals from forming inside of plant cells.
In a flash freeze like we experienced this week, the plants don't have time to react and the water inside the cells of the plants freezes. Ice crystals are pointed and sharp and, since water expands as it freezes, push outward puncturing the cell walls as they go. Enough cell damage = tissue damage. Enough tissue damage = plant damage/death (sometimes partial, sometimes total, depending on the plant and the situation). Leaves usually take the brunt of the damage, but in some plants, stems may also be damaged. This damage usually takes longer to show up; sometimes months.
The key, now, is patience. Do not prune. Wait. The full extent of this damage is not going to be known until spring, and maybe beyond. If the stems have not been damaged, many plants will leaf back out. Even so, it may take a full season or two for some plants to recover.
We will also see damage in deciduous trees and shrubs. You can count on it. Thin-barked species like crape myrtles, Japanese maples, and others may have suffered damage that will be completely hidden until they try to leaf out in spring. Small stems, less than 3/4" or so in diameter may have been killed outright in more tender species. You may also see bark damage in the form of splits or cracks running up the stems/trunks, or dead patches of bark (which usually turns black). Depending on the extent and location of this damage, you may see the death of some or all of the stems or, in the worst case, tree trunks and the resulting death of the tree.
In a similar situation in Tennessee in the late 80's, mature 'Nellie R. Stevens' hollies (and many other species, both native and non), crape myrtle, etc. were killed completely to the ground. Don't be surprised if it happens again, especially on recently planted and/or younger and less-established plants.