My Daughter went to live with her father 2yrs ago Now I want her back We hav never been to court for custody?

My daughter father & I have never been to court for custody.My daughter has always lived with me until 2yrs ago she wanted to go stay with her father. At the time he was doing great so I let her. However, I done so with the understanding that I could get her anytime I wanted to. I have recently found out that her father has been using drugs & had to be taken to the hospital on several occasions for over dosing. My daughter told me yesterday that Social Services came to her school and talked to her but I was not made aware of this until now. I have my taught for the summer and do not want to let her return to her fathers. Since her father & i have never gone to court for custody, can I keep her without him being able to come take her from me. My main concern is that since she has been living with her father for the last 2yrs does that give him grounds to come take her from me if I try to keep her. I know i need to go to a lawyer to begin the custody battle but in the meantime I do not want her going back to her fathers. What are my options & my rights? Please help!