texting/messaging not working

[mclone325]

So I just got an iphone 3gs and everything is working great, phone calls can be made and received and everything, except I can't seem to send messages/texts. I write a text, it says sending, but apparently the texts never get there and the texts sent to me never arrive. I have tried texting both another iphone and phones from both at&t and other service providers. but neither are working. I am not new to at&t as well, I switched over from an old phone to an iphone. I also previously had a texting plan with at&t. If anyone knows what might be wrong I'd be grateful for the help.