What should i have done?

add her on facebook and msg her or if you have her number, text her. just let her know you're thinking about her and tell her how you regret not asking her out. maybe she felt the same way and was waiting for you to ask her out. I mean, the worst that can happen is her not message you back. so why not take a chance? if you don't, you will regret that too! good luck! :)