DON'T LIE!!!!!!!!!!! Whatever you do, don't lie. Don't make up stories that aren't true, she will find out later and hate you for it, not to mention never trust you again.
If you handle it very matter of fact, she will be fine, in fact she will handle it better than you are at the moment. People don't miss what they don't have.
After divorcing while my oldest three(now grown)children were young, their father became a dead beat dad, moved across country, worked under false names, etc... to avoid child support. He would call the kids once or twice a year, make promises he never kept, tell lies, etc... Yes, it ripped my heart out. They got sad once in awhile, but for the most part they didn't even think about it.
I was always honest and told them their father and I could never get along again, but he was their father and if they could have a relationship with him they should.
Two of the three still don't speak to him. When we all moved across country, and they were young adults, I hunted him up for them. I took two of them to visit a few times. (One of them had no interest in ever seeing him again) Of those two one never saw or spoke to him again, and the other speaks to him once or twice a year. They don't hate him, but he is a stranger to them and they don't go out of their way for him. As they became adults and two of them parents, they understood the whole situation.
Keep it simple, some kids have daddys around and some kids don't. She has a mommy who loves her very much. She will be fine.