Why does my samsung moment android keep popping up a failed message alert?

Everytime i send a text message it tells me that it didn't send. buit then the people i am havbing a conversation with reply to the message which waasn't sent? Not only does this happen during text messages but it won't allow me to connect to the internet either. Someone please help me. I have like 4 application killers and virus detections on there and they all say it's clean. Does anyone know if this is a settings issue and how i can fix it?