I have had a crape mytle (Lagerstroemia) for 5 years. This year the leaves are not on all the branches, just a few.
Crape Mrytle trees like a soil ph of 5.0 to 6.5, and, therefore are acid-loving plants. Do not apply lime to the soil since it will make the soil alkaline. Also, lime, unless it ia hydrated lime, takes years to change the soil chemistry. You will not notice any changes this year. A modest application of fertilizer will help. If your plant doesn't look good this year, it is due to a very difficult winter. Give the plant time and it will recover.
You can get a soil test kit from Cooperative Extension in Philadelphia. The test will tell you whether the soil needs any adjustments.