I asked this girl out who I’ve known for years on a friendly day out, she asked if it was a date “Oooooo, is it a date?” (with a wink), she is attractive so I said “sure! So is that a yes?” then she just disappeared. She came back a week or so later to wish me a happy birthday, I expressed my gratitude towards her and asked if she was still interested in going out… she disappeared again
I think you caught her by surprise twice. Her asking if it was a date was probably more of a joke and now she doesn’t know how to tell you no and rather disappears on you.
Don’t bring up the date next time you talk to her, let it go back to normal and maybe she eventually bring it up herself