leaves falling off
It is not unusual for roses to lose their leaves during the summer. They prefer cooler weather and when it gets hot they often shut down. They drop their leaves to conserve energy. If you haven’t already, consider adding organic matter and fertilize your plants. Check out this guidance: https://pitt.ces.ncsu.edu/2013/04/roses-4/
That being said, your situation sounds pretty severe, so it is possible that the leaf drop is due to a black spot infection. This disease is caused by a fungus that is more common during wet conditions (like we’ve had recently). As its name suggests, black spot shows up on the surface of the leaves. As the spots get larger, they become surrounded with yellow, eventually causing the whole leaf to turn yellow and fall off.
Check any remaining leaves on the plant for dark spots with yellow areas around them. If you find these spots, You can use fungicide sprays to control black spot effectively. I conditions remain wet, you may need to spray weekly with one of the following fungicide sprays: chlorothalonil, mancozeb, or copper fungicides. Follow the label directions.
Here is a good resource for controlling black spot and other rose diseases: http://www.clemson.edu/extension/hgic/pests/plant_pests/flowers/hgic2106.html