What if a girl sends me a message and doesn't see it? I sent it two days ago. I didn't pay attention to the message and said wish me a merry Christmas. I didn't see it or reply.

It depends on whether you still like her or not. If you like to call seriously and say that you just saw it, she will send you a message saying that she is interested in you. You can take this opportunity to apologize and invite her out for dinner or something. If you don't feel anything now, just reply a message and explain the situation, so you don't have to be so entangled, right?