make an appeal

If someone reported you - it won't matter when you got banned. You have a few options here, either appeal if you know what you did, or ask @Vanishment what you actually did. It should say what you did within your ban message. I doubt @Vanishment isn't refusing to tell you what you did wrong.

Actually, I was afk and didn't see the conversation until now.

So answer >:(

I already answered him in conversation.

The moderator didn't have to witness it for you to be banned for it. The report section exists, they might have just seen the report and banned you while you were asleep. Just because you weren't on at the time of the ban, that doesn't mean you're
innocent.
@Vanishment Should tell you why you're banned, but it should say your reason for being banned in the ban message.