I just found out the mother of my child puts her in her room in complete darkness to punish her,what do i do?

we are not together and she has all the custody and i havent been able to pay child support for four months and shes allready taking time away that i can se my child so i dont want to piss her off but this just seems so wrong to me,my little girl is only four and shes afraid of the dark so im so worried about what else shes doing i have to do something but what?she keeps her healthy and provides for her but the punishments she gives are way to harsh for someone so young.I just found out my daughter was mine 5 months ago cuz of her moms lies so im new at this please help.I feel so helpless and am worried about my little girl.her mom has charges against her past meth use and is on a butt load of meds for mental problems so shes not wright in the head