Not enough information. Make and model of the phone? Version of Android? Text app you're using? Type of phones you're sending texts to (Android, Windows, iPhone)? Have you checked with your carrier to see if a) your account is provisioned properly and 2) if they're having any problems with the tower you're connected to? Have you recently switched from an iPhone?
"Why is X not working" is the same as asking a mechanic "why is my car not working".