The girl sent me a message saying that she didn't know how to reply. What should I do?

I don't know what specific questions you asked that girl to make her so embarrassed. You can take the following measures to alleviate this embarrassment:

1, give her more time to think about this question and come up with a suitable answer;

2. Give up the answer to this question and don't force others to do it;

Change the subject, cover it up, and ask again at the right time.