Betsy:
I've read CK's answer, but I think your tree is suffering from Dutch Elm Disease. DED is a fungus infection carried by the larvae of bark beetles, and is nearly always fatal. Leaves and branches wilt and die from the fungus. Onset can be in late spring.
For treatment, look in the yellow pages under Arborists. An arborist can evaluate and treat the tree. A local nursery or Extension service should also know who to contact in your area.
Treatment may consist of removing diseased limbs, applying insecticide or injecting fungicides. If the tree is too badly diseased, removal may be the only option.
