Message sending failure.

Hi all, I'm having problems sending messages. I keep getting message sending failure every time I try. This is for both sms and iMessages. I'm on PAYG and have free messages, also still fails with steady internet connection. Have tried basic fixes via Google search and nothing works, this has been a problem for a while but normally its ok after a day or two. not this time though and I'm reaching breaking point now. Any help greatly appreciated.