It's very hard to say when all we know is that it doesn't work (so no other information which would give us a clue as to why this is). And I'm afraid that phone runs Windows Mobile 5, not Android, so I doubt that many of us know much about it (I've never actually used that system).
I'd try your SIM in another handset and see whether you can send texts then. If you can, it's something in the phone settings - in which case, since I don't know this system, all I can suggest is a factory reset. If you can't send texts with a different handset it's likely to be a problem with your account, and you should speak to your carrier.
I'm sorry, but that's all I can offer.