after oil service oil in the cooling system

Hello I have a question here….. I was on Thursday to change oil and today I had to find out that I hardly have any left The lid of the oil filler plug was not screwed properly on it and thus the oil spread nicely in the engine room and on my driveway …..but at the short check, i.e. lid again properly on it, see if there is still something going on somewhere, I had to find out that I apparently have oil in the cooling system and that was not before. I can’t imagine the “suddenly” my cylinder head gasket is giving up the mind, but how can it still get in there? Can something come in somehow through the senseless distribution of the oil, or the crankcase ventilation or about…I don’t know. Would I be happy about a good tip LG Die Potato