I sent a message to a girl I like, and she didn't reply to me for a long time. Did she just forget me and take me seriously?

This state of yours belongs to the consideration of gain and loss. Simply put, you are too nervous. It is very likely that the other party is at work or in a meeting, receiving important guests, listening to the leader's lecture or dealing with very important things, so he has no time to look at his mobile phone and can't pay attention to this short message you sent. I suggest you contact him later.