and welcome to the forum.
The causes of chlorosis ( loss of the green coloration of the leaves, caused by a lack of chlorophyll) usually fall into one of 5 categories: plant pests, diseases, nutritional deficiencies, light problems, and water problems.
Here is an article which should help you trouble-shoot. https://gardeningvibe.com/cucu...
While your problem may be any of the five, the increased heat and missing watering that day may have precipitated the problem. Cucumbers need stable moisture once they are producing. If it varies too much one way or another, that's when problems can start.
I suspect from your pictures that your cucumbers have Downy Mildew. Perhaps another member will concur or suggest something else. It's good to get a few opinions, since these problems are not always easy to trouble shoot.
Egg shells are not going to help at this point if you are trying to introduce calcium. It takes a long time for the calcium in egg shells to be available for plants. Egg shells are best crushed, ground as finely as possible, and placed in your compost pile where they will work their magic over time and the calcium will be available as you use your new compost.
If you suspect a nutritional deficiency, use something like Blue Planet Nutrients CalMag Pro Formula which will address several nutritional deficiencies at the same time. https://www.blueplanetnutrient...