i want to know the possible harmful effects in absence of FBS for 72 hour?

Without the proper growth factors and things like transferrin the cells won't grow even if you get them to attach.  There's a paper that describes an FBS-free growth medium for MCF-7 (link 1) that you can use as a starting point in exchanging growth factors. There are also several commercially-available serum-free media options available (link 2).

http://www.nature.com/nature/journal/v281/n5730/abs/281388a0.html

https://www.thermofisher.com/de/de/home/life-science/cell-culture/mammalian-cell-culture/serum-free-media.html

Does MCF-7 still able to attach after 72 hour?