Sounds probably that the head has warped from overheating and so the gasket is no longer sealing the cylinders. This will mean that pressure from the pistons is free to vent out into the water chambers which surround the cylinders, hence the blowing into the coolant reservoir.
Worst case scenario is that the pistons are scored, which will mean an engine write-off, as the cylinders will be scored too and it's unlikely a re-bore will work. Best case scenario, you'll need a head-skim and new gasket, which although expensive, will not be the end of the car.
Cheers,
Mike
PS Other thread deleted, as there's complete duplication