I said something wrong to my friend, and I didn't find it at that time. When I found out the next day, I texted her to apologize, but she didn't reply. What should I do?

If you say it, it's like spilled water, and you can't get it back. Don't feel guilty, because everyone will say the wrong thing sometimes. If you should break your word, you should lose a friend, because a true friend will not be angry all the time, and he (she) will give you a chance to explain. However, you were wrong before. You must find him (her) yourself and sincerely apologize to him (her). I believe a true friend will forgive your carelessness.