こんにちは、Aceさん。 When you reply to someone, just say "ひさしぶり" in the same way. If you want to say something more, informal「わー、久しぶり。元気〜？一年ぶりだね！どうしてた？Long time no see.One year since we met! how have you been?」etc.. formal「お久しぶりです、お元気でしたか？一年ぶりですね！いかがなさっていましたか？」etc..

You can simply answer with “久しぶり.” Or you may say “久しぶり! 元気にしてた?” to ask your friend how he/she has been to start up a conversation to share your life with each other.