which one is correct?

[Zoe2008]

Please reply to Helen

Please reply Helen

[susiedqq]

What do you want to say?

Do you want to tell people to contact Helen? Then that is "Please reply to Helen."

Are you talking to Helen and want her to answer you? Then you say "Please reply, Helen."

[Raymott]

What do you want to say?

Do you want to tell people to contact Helen? Then that is "Please reply to Helen."

Are you talking to Helen and want her to answer you? Then you say "Please reply, Helen."

And to venture further into the realms of unlikelihood:
"Who should I ask to the dance? Please reply Helen!"