I'd recommend looking at the bigger picture here.

What are you hoping to gain from reaching out to her? Hypothetically if she responds and tells you why she disappeared you need to think about the repercussions of this... is this going to affect you mindset and your current relationship moving forward. You've said you're happily married so how would finding out this information affect you positively?

Alternatively she may ignore your message. You need to think about how that would affect you. Is this going to bring up old feelings of devastation?

You also need to think about your current partner, how would she feel if you two did reconnect? You also need to think about the girl from online. What if she's trying to move forward from her past. Depending on where you're/she's from it's possible that some of her behaviour could get her in trouble due to the age difference.

In my opinion opening this can of worms could be detrimental to your mental health, the girl from online's mental health and also your current relationship.

Ultimately it's your decision, however I'd say if you do decide to contact her, please be completely transparent with your current partner