Hi, CeeBee.
It was probably "O Come all Ye Faithful", which is a traditonal Latin tune "Adeste Fideles".
http://www.bellaonline.com/articles/art7292.asp
Adeste Fideles History and Lyrics
Adeste Fideles, also known as O come all ye Faithful, is a traditional tune first written in 1742 by John Francis Wade in France. It was translated into English in 1852.
Adeste fideles,
laeti triumphantes,
Venite, venite in Bethlehem!
Natum videte,
Regem angelorum
Venite adoremus,
Venite adoremus,
Venite adoremus
Dominum.
Cantet nunc Io!
chorus angelorum;
Cantet nunc aula caelestium
Gloria, gloria,
in excelsis Deo
Venite adoremus,
Venite adoremus,
Venite adoremus
Dominum.
***
O come, all ye faithful,
Joyful and triumphant,
O come ye, O come ye to Bethlehem.
Come and behold Him,
Born the King of Angels!
O come, let us adore Him,
O come, let us adore Him,
O come, let us adore Him,
Christ the Lord.
Sing, alleluia,
All ye choirs of angels;
O sing, all ye blissful ones of heav'n above.
Glory to God -
In the highest glory!
O come, let us adore Him,
O come, let us adore Him,
O come, let us adore Him,
Christ the Lord.
Yea, Lord, we greet Thee,
Born this happy morning;
Jesus, to Thee be the glory giv'n;
Word of the Father,
Now in the flesh appearing,
O come, let us adore Him,
O come, let us adore Him,
O come, let us adore Him,
Christ the Lord.