Why is it that when one boils beans for lie 15mins to 20mins and one leaves it to cool down/get cold, it never get cooked again. It happened to me in school then. I was cooking beans with stove and my kerosene got finished. Before I could get it back to cooking because I was broke then, it was already getting cold. NEPA restored power later and I kept this beans on my electric cooker for almost 2hrs but it was just the same as it was before I continued cooking it.
I was told it was never going to get cooked again but I didn't believe it. I later dispose it off.

That was around 2010 and I never tried or experienced it again since then. But is it always like that?