Probably a cold night before they were fully hardened off - or even afterwards. They are sensitive to low temps.
They may recover with a bit of TLC. You don't say where you live on your profile - this is always helpful in answering questions as the British climate caries such a lot.

As Steragram said, it could be cold temperatures that's causing the problem or even over watering them

It could also be a nutrient deficiency. Try adding some Magnesium Sulphate (Epsom salts) to the water at the rate of a teaspoonful to a gallon. Or give them a foliar feed with Phostrogen or Miracle Grow.

Over watering can do this.

Also from experience on my allotment this happens when growing the in those biodegradable pots. Quite a few of us used them last year and planted them out green and lush only for them to go yellow. We had to carefully dig them up, removed bio pots and replant. They recovered fully. We only found out because one plot holder ran out of biodegradable pots so had planted half in normal pots which were fine. I never use these on beans now.