Hi, It Was My 12th Week Pregnancy, Got Baby Heart

Hi, it was my 12th week pregnancy, got baby heart beat at 7 weeks 1 days... 6 days back went for normal and blood check up & every was perfect on that day... they asked me to come for scan on 11.1.2017. but after scan they said baby heart beat has been stopped and some water is stored around baby neck and one layer is created (Water layer)
around baby, but am really confused y baby heart beat stopped? what would be the major reason?
please let me know.
now doctor asking me to go under D&C.
this was my first pregnancy,only 7 months after my marriage.