Dogwood does not look good!
Thank you for your dogwood tree question. It would appear that your tree is infected with dogwood anthracnose, caused by a fungus that proliferates in cool, wet spring weather. I've attached a photo of a dogwood tree leaf looking much like yours, with a known anthracnose infection.
As this article indicates, (under 'Control") your tree may be saved by severe pruning of all infected leaves and stems. I cannot tell from your photo whether every leaf shows symptoms of the infection. As this source also tells, if the cultural and physical methods of control do not work, there are two fungicides available to the homeowner (Bayer Advanced Disease Control and Bonide Fung-onil Multipurpose Fungicide), but you have missed the 'window' for their application to be effective. All you really can do now is leaf and branch removal of infected tissue, and raking up leaves that still have the infection so your tree (assuming it survives) is less likely to have the problem next year.
I hope this information is helpful, and that your tree survives this very serious problem. Good luck!