Hello! I would greatly appreciate your(plural) help!

So my seedlings (plantlings? since they have 2 true leaves) are dying! I started a bunch of seeds - vegetables and flowers (pepper, cypress vines, zinnias, etc) indoors to transplant outside. They grew with gusto in the pots, but the leaves turned light brown (starts spotty, then spreads), soften, wilt, then die. The root is gone too :( I've been starting seeds like this for many years, but this is the first year that almost all of my plants died!

I'm partially suspecting the weather - temperatures have been very fluctuating this spring in Central New Jersey, ranging from 80*- 40* all within a span of a week. I bring the plants indoors at night.

Any help / advice / tips would be greatly appreciated!