Well, it could be environmental (drought, flood, herbicide , etc.) or biological (insects or disease), or a combination of the aforementioned factors. It is impossible to determine "cause" without an on-site assessment of the tree or without submitting a sample to a university plant clinic for culture.
What should you do? Quite simply, you need to contact your local state Cooperative Extension office or a local certified arborist for assistance in your home state:
Virginia Cooperative Extension: http://www.anr.ext.vt.edu/index.html
Finding a VA Certified Arborist: http://www.dof.virginia.gov/mgt/arborist.htm