The mobile phone suddenly can't send text messages, and suddenly it can't send them today. No matter who you send it to, you will be prompted that "this number does not exist"

You can call your mobile phone card unit for manual consultation and inquiry.

I think it is possible that you accidentally deleted the service number that provides short messages. You can ask them to tell you again. You can find and enter the SMS service number, and then you can send text messages.

I used Unicom's card. It happened when I first bought it, and it hasn't happened since. There is another reason. When the network is unstable or you are in a place with poor signal, texting is not easy to succeed.

If you haven't tried these methods, you'd better ask where there is a mobile phone.