There are many reasons people fall away from being an active Christian: some people get so busy with life they just slowly fall away; some have been hurt by someone in the church; some are drawn away by worldly concerns or even a sinful lifestyle.  Whatever the reason, as a baptized Catholic you can always return to full participation in the life of the Church: Jesus is always ready and willing to accept you back.  The only thing needed is your desire and willingness to take the first steps.  see Mathew 13:1-9