the only thing that i can really think of is that it didn't flower long enough maybe, and the fact that you didn't give it any nutes other than the mg soil. or maybe you used a bag seed that just had bad genetics and wasn't too strong on the potency front.
i would say start again.. go to a trustworthy site like the attitude or any other with a good rep here on riu, and order yourself a nice sounding strain that is mainly indica dominant.. ask for peoples ideas about what is a good strong strain for newb growers like yourself..
sorry to hear about your buds, but if you learned something along the way that you can use for a future grow, all is not lost..