My zucchini plants produced beautifully for the first part of the season,...
Your description of problem is a bit too general for a single diagnosis. The following links may be helpful for matching to your specific symptoms:
The Penn State Extension and the University of Minnesota Extension have overviews that may be of help:
More information on bacterial wilt: http://u.osu.edu/vegetablediseasefacts/cucurbit-diseases/bacterial-wilt/
Once a specific diagnosis is made, control measures may be recommended.
I hope this is helpful.