My tomato plants have already died. They turned yellow and then brown. I did everything the garden center said and they kept getting worse. Now my zucchini, peppers and cucumbers have these white spots on them. I was going to plant them today but not sure if they will make it. The last 3 years have been a fail for me with the garden. This year I tried by seeds and failed. Please help

Powdery mildew is one of the most commonly occurring plant problems. It is a fungal disease that affects plant leaves and stems, coating them in what looks like a white or gray powder-like substance.
Combine one tablespoon baking soda and one-half teaspoon of liquid, non-detergent soap with one gallon of water, and spray the mixture liberally on the plants. Since Powdery mildew stays in the soil via spores, you may want to change your garden location to a different area next year.

Cold damage?

We need more cultural information...
as sue suggested maybe they were outside and experienced a light frost, or... maybe you put them in the bright sunlight after being inside and they couldn't handle it.
when bringing tender plants outside, there's a process of acclimation... where you put them in part sun and gradually increase the amount of sunlight.