Your Kwanzan cherry tree is suffering from brown rot fungal disease.  This disease has just recently jumped to ornamental stone fruit trees and the Kwanzan has shown to be highly susceptible to the disease.  This disease typically attacks fruiting peach trees and destroys the fruit.  Consequently, it is necessary to enter into a regular fungicide spray regimen in order to control it and enjoy a fruit crop.  Ornamentals, on the other hand, may look ugly for a time, but the disease is prompted by cool, damp spring weather, so the incidence should diminish as temperatures rise.  You should rake up everything that falls from the tree and prune out any obviously dead wood.  We anticipate that the tree will survive and recover.  You could spray a fungicide next spring, but it may not be practical to do so.
LS