My Daughters mother has given up custody of our daughter and I was Not Notified.

[Lorenzo Davis]

I am single and helping my mother with our other two children my stepdaughter and biological son. Their mother is supposed to have custody of our other daughter and son both which should be biologically mine. I have recently discovered that she still has custody of the son but our daughter is not with her. How do I go about legally obtaining custody of my biological daughter and why was I not notified about any and all custody issues