has no power what so ever , new battery ? ?

i have a craftmans rider 15.5 hp . older mower . it kept draining battery so i got new one did the same thing , my son inlaw said take n unplug some wire well the dau did so n then stuck knife in the end of plug n it wont start at all nothing wont fire any thing . what did she do ??? how do i fix it??helpppppp