Most Helpful Girl
Look, you could have never sent it and always wondered what could have happened if you sent it. The worst thing that could have happened is her not replying but at least you know now. Try to find the positive in it and just acknowledge the outcome and move forward from it. Don't beat yourself up about it too much.
Most Helpful Guy
It wasn't stupid, it's never stupid to wish someone a happy birthday. You might say that it would be naive to have expected much to come from doing so, but you can still be happy that you were nice enough to do it. If she doesn't respond then fine, life goes on. You've still done a decent thing.