If you've mashed and sparged, surely the grains will be spent, (and ready to be thrown away), and you will have collected the wort in some other container?
Due to the temperature of the mash, there would be some pretty lively bacteria in your wort, and unless you boil it straight away, they'll continue to multiply if left overnight, producing the hideous smell mentioned.
Having said that, I don't know what the effect might be of storing the container in a REALLY cold fridge (near to freezing, in order to stop the bacteria from multiplying) would be. It might be worth an experiment some time, if you're desperate for time.
PB