Many plants have winter damage after this past winter's severe temperatures, especially plants that are fairly tender (more Southern) plants to begin with.
Let your crape myrtle complete leaf out so you can easily see what parts of the wood are still alive. Prune off all the dead wood. Do not try to fertilize it back to health. Fertilizer is not medicine. However, you can lightly fertilize it once, in case the soil is missing any nutrients.
The most important thing will be water. Be sure it gets watered during dry spells. Plants like soil about as moist as a wrung out sponge. Be aware of droughts--they can occur in spring and fall as well as summer. In fact, last years fall drought left soils dry going into winter and that is what caused much winter damage to be worse.
New plants should get supplemental water during dry spells for at least the first 2 years so the roots can get established. They like about 1" of rainfall a week.