What do I do? I still miss her..
Assuming it's genuine, not a bot, you follow through, that's what.
Whether she still misses you or not, she's curious enough to want to find out.
What you don't do, imo, is take any notice of the comments on this thread (including mine!).
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One my ex's contacted me at the Millennium. She lives approximately 24 hours traveling distance away. Since then we've emailed each other maybe once a year. She has sent me ecards for my birthday. Which reminds me I forgot to wish her a happy New Year at the start of 2018. (And in case you're wondering, Mrs Bnc doesn't mind. ex, who has never met Mrs Bnc, includes sending her (ex) regards to Mrs Bnc.
I still keep inn touch with the first Mrs Bnc. First contacted me a couple of years ago (wanting some advice on selling her father's house), the first contact for ages. After a flurry, including my sending her father a birthday card for his 100th, our contact has settled down now to birthday and Christmas greetings.
Personally I think it's nice to keep in touch with those I've known and been friends, etc with over the years. Whether you want a renewed contact to go more than one stage further depends upon your and the other person's circumstances.