Do you have a photo thundercat? Could you perhaps frame it (or find one on the dreaded fb, and print it off - not strictly ethical, but heyho) and give it to her - a talking point, the start of a chat about daddy, who he was and what he liked.
You can be honest and shruggy without being negative. Simply saying "i don't know why he chose not to be part of your life, and I don't know what he is doing now" won't satisfy her curiosity, but it will mean that she has an outlet to ask. You could give examples of why he perhaps made those decisions, or what he might be doing now, based on your past experience of him. He wasnt a bad person (I am assuming from your posts) he just wasn't the type that wanted a family - it ISNT about her, as she wasn't born, he's never met her, but if he had been around he would of course have loved her etc etc.
If you cannot find photos of him, could you take her perhaps to a restaurant that you and her father went to together, and tell her that you have taken her there because this is where you came with her dad, and you'd thought she'd like to see it - or such other place ( a local park, or a street you once walked down, your old house etc etc)
anything really just to get the opener. Good luck, yoll be fine at it, I am sure.