If a catholic Recieves the Eucharist whilst being 1.) does not know he/she is in mortal sin and 2.) If they are in the state of sin. What happens if they don’t know that taking the Eucharist is also a sin whilst in the state of mortal sin?
If they don’t know that they’re IN mortal sin, it may not be a sin at all (though it should always be Confessed). It’s at minimum a venial sin if they know they’re in mortal sin even if they really don’t know the rules on the Eucharist, though I would argue it’s incredibly hard for a Catholic to not know (vs. ignore) the rules on receiving the Eucharist.
If you don’t know you are in the state of mortal sin, you are probably not in the state of mortal sin. You can’t commit a mortal sin without knowing that it is a mortal sin. So it is possible to commit a gravely disordered act and not have it be mortal sin.
If you were later to realize that a sin you committed in the past was intrinsically morally grave, you should immediately go to sacramental confession. However, in order for a sin to be mortal, the three conditions must be met at the time of the commission of the sin.
As Catholics, we are called to educate ourselves to prevent putting ourselves in morally dangerous situations. Ignorance is a weak defense, and in itself can become sinful (willful ignorance is a sin).
If they grew up in an enfeebled social atmosphere where some of this stuff was swiped under the table, they probably have little or no culpability over the sacrilege, but as we get older at some point we need to wonder what it means to be what we claim to be. Willful ignorance is not going to protect us. There are, perhaps, a great many people that know little about their faith because they have an intuitive feeling that they won’t like what they will find.