I don't know if I should enter her life again.
saying hello on FB is not entering someone's life
you don't know what's going on with her these days - she could have an entirely different life since you were last in touch
but yeah, say hi, it really isn't that big a deal
I've made contact with people I hadn't been in touch with for over 20/30/40 years. No one's life was changed as a result of the contact but it has been fun catching up.