I do see the tiny brown dots on the stems and because from the photo I see part of the plant is still healthy I would suspect vine borers as well. Try to cut out the entire stem with the punctures to destroy the insect that is hiding inside. Some gardeners even use a stiff wire poked into the stem to try and stab and kill the larvae of the moth that is doing the damage. You can attract adult moths that are flocking to your plants by setting a yellow dish or bowl near your plant with a bit of soap in an inch of water. The moths are attracted to the yellow color  and fall in and drown due to the soap. You can also try floating row covers early in June to keep the moths from laying eggs on the plants. You do need to remove the row covers when the squash flowers so the bees can pollinate. Finally, you may need to replant a second crop in July. The borer moths will be done laying eggs so a late crop may escape the borers.   Don't give up as many times zucchini will be able to outgrow this problem and you may still get a good crop.  One more thing - rotate crops as the borers may overwinter in the soil. Keep growing, Marianne Binetti