Why did my whole garden start dying?

The only times I heard of something like that was when 1- herbicide drifted over into an area that it wasn't intended for and 2- water soluble fertilizer (like Miracle Gro) was broadcasted onto the soil. The granules were too potent and the plants receive fertilizer burn. If you didn't have any type of that activity going on, do you have any neighbors that have that going on? If neither, you might want to start thinking who might want to trash your garden.