Crush B'day next week !

I’ve had a crush on a girl from my class since we were kids—we were actually together from nursery until 10th grade. After school, our paths diverged; she pursued medicine and is currently in a Government Medical College (GMC). We are in the same group chats, and I have her number, but she’s usually inactive. Her birthday is next week, and I want to use it as a chance to wish her and restart our conversation. She knows I like her, and back in our school days, I’m pretty sure she had feelings for me too, but I’m not sure where we stand now. I’m honestly a bit afraid to text her. What should I say when I wish her? How do I keep the conversation going without it being awkward? Also, what kind of reply should I expect given her busy schedule? Would love to hear your opinions!