[Rack n' Roll]

I have this phone & it always worked okay, but last week I sent one message and it kept on sending til all my credit was gone & I tried everything to make it stop. I took out the battery thinking it would stop, but as soon as I put it back in it continued sending. So eventually I ran out of credit & then the next day I topped up & I am now unable to send any messages. Everytime I try it says 'SMS Send Failed' I am still able to make & recieve calls & the o2 service centre number is correct. I only have this phone a while & I really love it, apart from this slight fact.

Does anyone know how to fix it?

Thanks