Actually it is needed to add one line “playButton.interactable = true;” as below for the button to be interactable after you minimize you app and wait more then “energyReady” time. Otherwise button will remain inactive but with full energy - untill reload.
if(DateTime.Now > energyReady)
{
playButton.interactable = true;
energy = maxEnergy;
PlayerPrefs.SetInt(EnergyKey, energy);
}