@Samlish - I don't want to make you feel worse after that ordeal (my day was horrible, in case that helps), but you can't re-adopt a dog if it has been taken away by the pet care social worker. Or do you have a mod in play that actually does make that possible? Generally the town feels that if a sim is not capable of keeping the pet healthy and happy, it does not wish to see them reunited.
Or perhaps I'm reading too much into this and you were playing TS4, but posted on the wrong forum board? :/