Text Messaging

I have had my Nokia N8 since October and up until the beginning of this week have had no complaints.  Now when I try to send text messages I get a message saying "unable to send message to.....", but the message goes anyway.  It doesn't appear in my sent box but remains in my out box as if it hasn't been sent, so unless I get  a reply from the person I sent a message to it would appear as if the message hadn't gone.  I don't know why this has suddenly started to happen.... any ideas anyone???