Why? Why would you do that? Now you've put yourself in a position of having to wait for her reply and you can't think about anything else until then. Never tell someone you like them until you've been out on a few dates. Without dating her, how to you even know you like her anyway? You're trying to skip the first couple important steps here.

That's exactly the stupid thing you did when you said it was a joke... The fact that she said "what a nightmare" (I can even imagine how she said it, and she must have smiled) is most likely because she already felt that you liked her, and this confession may have been unnecessary, maybe she did not want you to like her, or, on the contrary, she is also very sympathetic... But here by saying that it was all a joke, you yourself spoiled the situation, that is, imagine:
- I like you!
- Well, ok!
- And I was kidding.
It's kind of stupid, isn't it?! I don't think you'd like it either. So run along and apologize. And add that you like her even more now. Otherwise, you'll lose her!
I read this on the website in an article about restoring relationships.

I told the girl I liked her. In response, she said to me: "What a nightmare," and I said with a fright that it was a joke, and now she does not communicate with me. What to do?