I had a lot of green grape clusters on the vine, but they have begun shriveling and not ripening. What have I done wrong?
Why Are My Grape Clusters Shriveling and Not Ripening
There are several possible diseases. More information here:
Two diseases found especially in cool, moist weather are botrytis
and downy mildew https://www.gardeningknowhow.com/plant-problems/disease/downy-mildew.htm
You can take some fruit and leaf samples to the county extension service for a positive identification. This link will help you locate the closest one: