Answer
Noun and する verb
へんしん
やばい、返信するのわすれてた。ちょっと大変なことになってしまうかも。
Oh no, I forgot to reply. What have I gotten myself into?
き読になったのに、いつまでたっても返信が来ないんだよね。
It said "read," but I never got a reply.
お疲れ〜。返信したつもりだったんだけど、どうやら力尽きて、送信ボタンを押す前に寝てたみたい。ごめんね、許して〜。
How’s it going? I thought I sent a reply to you, but it seems that I was too tired and fell asleep before pressing the ’send’ button. Sorry. Forgive me.
You gotta believe it will return. What are you believing in returning? The reply. You made a request for something (come up with something you want really bad but have to ask for). Now you wait for the reply.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.