Well, if you weren't there for the birth, then there is little info for us. More than likely, if the tank is heavily planted, she would not have managed to eat all of the fry, depending on the amount of time you were gone...
She might have abandoned the birth, I know a lot of first time mothers do. It was probably something to do with the birth.
I am sorry, but there is little we can do to diagnose with no pictures/little to no info
I suppose whats done is done. There is nothing you can do now. However, to reduce heartbreak in the future, I would recommend just purchasing male mollies.
If you don't know how to tell the difference, I know how to....so just ask ok?
I am actually skilled at it if I do say so myself