Text could not send, what is wrong with my phone?

I have a sony ericsson xperia x10 mini, I recently added a number onto my phone and texted that number for the first time two days ago, it sent fine and i received a text fine, but then today when i tried to reply to the text I received it would not send, I tried again, i re-wrote the message, i turned my phone on and off, I took the sim out and put it back in, but it still would not work. I can send texts to anyone else and it will send but just not to this one person, would could be causing this? please help me