help bread second rise is flat?

It sounds like your yeast has died somehow. It could also be that your yeast didn't have enough to eat during that time, so stopped dividing (and was therefore flat). It's perfectly fine to let breads rise twice, and can result in a lighter, fluffier loaf. The yeast should live long enough to tolerate that. Nothing you can do but try again and hope for a better batch of yeast.

I never knead the dough twice. After the first rise, all you do is punch the dough down, then shape the dough to fit your loaf pan, or into rounds, etc. Cover and let rise, then bake.

Sounds like the yeast has died.

Either it was not fresh enough,

or the temperature was wrong.

Dough has to be kept warm to rise.