zucchini leaves suddenly yellow or the plant suddenly dies....CHECK
upon inspection, the base of the plant has been chewed away....CHECK
you can see the nasty little white borers right there in your plant stem or eggs under the leaves.....CHECK
Several Bug insecticides are available in powder or liquid spray. Look for one that lists vine borers on the label. Apply every 7-10 days for two or three treatments. You can also bury the stem under soil after killing the borer and the plant may be saved.
Recovery & Prognosis:
Caught soon enough, it's possible to save the plants.
Next year I'll be on the lookout for these little buggers much sooner!