It sounds like the engine has fairly recently overheated causing the head gasket to fail. This was possibly caused by a crack in the head or a warpage of the head. You can take off the head and have it checked and surfaced at the auto parts shop. Put on a new gasket and things should be back to normal, however, I would be looking for a reason as to why the vehicle got hot. I.E. Crack in the radiator, leak in hose, bad thermostat, etc.