What happens if marriage doesnot take place after engagement uner Hindu marriage?

There is nothing one can do about it except move forward.

Not to disagree but I think they want to know what legal or physical ramifications can happen to them in their country and faith when they do not go through with a marriage.
Not all countries conduct their marriage traditions the same as the U.S.A.

I should have mentioned this... I am myself a Hindu and know the laws and traditions of my faith.

As far as my knowledge goes, there is nothing a person can do in this situation. There might be a slight social stigma attached. But yes, there is a lot of heart ache and emotional stress.