Its like most other things you cook. You wouldnt be surprised if you part cooked an egg or a piece of fish and then found it wasnt quite the same when you came back to them a day or so later and tried to complete the process. Roasts will sometimes stick if the temperature drops. Letting it go completely cold almost certainly means you have produced something good for the garden and not much else. Providing you were working with a small roast and the beans were not Yemen Ismaili or something even more valuable, you have learned something useful and it probably cost you more in petrol to go back to Bunnings than the beans were worth.