How and when will it be the right time to talk about my doughter's real Father?

Dear Community.

The issue is a bit complicated.

My daughter is currently 5 years old (she will be six in 2 months) and She is very sad, because she thinks that my "exhusband" is her father, and he has stopped visiting her, calling her etc... We have been divorced for quite some time, actually for 10 years, together we have a son that is now 15 years old, and we stayed close for his sake, however the dependency became unberable and I had to leave due to behaviots that were no longer healthy for anybody.

Now my daughter is sad because she thinks that is her real father, and that she kind of has abandoned her. It breaks my heart everytime she gets like that. However I do not know when or how to tell her about her real dad.

The real dad, I have not been in contact with him since she was 5 months old, and he has not looked for us either.

I need help in this topic, I hope somebody can give me the right advise.

Sicnerely
Madam B