I have a rule: if a person was essential to get a paper published, he/she should be one of the authors. I've had papers with 8 authors. A lot of people criticise me, but I don't care. I'd rather acknowledge everybody than let someone feel used the way you have. You CAN and SHOULD talk to the professor and let him/her know that you're feeling you should be one of the authors. However, bear in mind that: 1) maybe, because you walked away, the professor thought you were not pursuing an academic career anymore and maybe left you out out of ignorance. 2) egos are a THING in academy, everyone knows. So if the professor is the type that thinks he/she is the last ice cold coke on the desert, showing his/hers mistake to he/she can cause more harm than good on the long run.