Here is an excellent article on why those in mortal sin cannot receive Holy Communion. Our Lady at Fatima taught the 3 children prayers to make reparation for sacrilegious sins committed against the Eucharist:
THE PARDON PRAYER
The angel who appeared to the children called himself the Angel of Peace and the Angel of Portugal. He taught them the following prayer:
My God, I believe, I adore, I hope and I love Thee! I ask pardon for those who do not believe, do not adore, do not hope and do not love Thee.
THE ANGEL’S PRAYER
Most Holy Trinity – Father, Son and Holy Spirit- I adore Thee profoundly. I offer Thee the most precious Body, Blood, Soul and Divinity of Jesus Christ, present in all the tabernacles of the world, in reparation for the outrages, sacrileges, and indifferences whereby He is offended. And through the infinite merits of His Most Sacred Heart and the Immaculate Heart of Mary, I beg of Thee the conversion of poor sinners.
THE EUCHARISTIC PRAYER
During the first apparition of Our Lady of Fatima, the children found themselves “moved by an interior impulse,” as Lucia later explained, to say the following prayer together:
Most Holy Trinity, I adore Thee! My God, my God, I love Thee in the Most Blessed Sacrament.
THE SACRIFICE PRAYER
The Blessed Mother taught the children to offer all their personal sacrifices to God by praying,
Oh my Jesus, I offer this for love of Thee, for the conversion of sinners, and in reparation for the sins committed against the Immaculate Heart of Mary.
THE ROSARY DECADE PRAYER
The Blessed Mother stressed the critical importance of praying the rosary daily for the conversion of sinners and for peace in the world. She asked that the following prayer be recited at the end of each rosary decade:
Oh My Jesus, forgive us our sins, save us from the fires of Hell, lead all souls to Heaven, especially those in most need of Thy mercy.
Read the article here: