Just because she was still in the store doesn't mean she wasn't busy, to be fair. That said, chances are when you kept trying to talk to her when she tried to avoid you, you made her angry with you. Stores also aren't generally the best place to rekindle a relationship. Next time, if you want to try again, just try asking if she wants to go to lunch or breakfast someday, and give her your contact info.

I feel your misery. I was in line at a grocery store and heard a familiar voice, so without thinking (she's really weird about having her name spoken in public and never has revealed her age to me), I said, "Hi, X." She had to have heard me but ignored me. Whatever. With your former friend, perhaps there's a reason you're no longer close. Maybe you grew apart, maybe she moved on. I know it hurts, but just let it go.

I hate running into people from my past. There's always a good reason I don't talk to them anymore. So I guess I would try to avoid you too OP.