Luck exist or not only god knows, Some times strange, out of way things happens with human which give a deciding turn in to fortunes of human ( windfall ).
But man should not believe on these things as these things make man dependent , weaker in his determination. His focus shifts from hardwork to easy return.
In our 6-7 std. there was a shaloka which means " soye hue sher ke munh main hiran nahin jaata "
or in urdu
every body knows " khudi ko kar buland itna ki har badi se pehale khuda khud bande se pooche bata teri raja kya hai "
and finaly what
Lord krishna had said " Krmanyev tav adhikarastu maa faleshu kadachan "
these things exists or not we should not believe in it. If our destiny is pre decided what is our purpose of taking birth . We are not programmed machines.
I personally think luck is like a wild card in God's hand which he rewards to a man based on his efforts
of past life or this life but it is in lieu of some good karma done by man.
regards