text messages

woke up this morning and tried to send a text to a friend and got the failure to send ! tried a few other people and got the same thing. Then I tried sending a text to my son who uses the same redmi 4s4x and the texts go through to him. I'm looking for some help if anyone can tell what settings to check or change. I'm 64 years young and not super smartphone tech savy.I can't remember if I made any changes in the settings or not.

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