You need to contact your mobile network. The settings might have been changed. How did you update? Did you use a leaked ROM? I think I'll wait until the official release for the S3 before updating. :-/
Open the default messaging app, hit the soft menu key (left of the home button), and you scroll down and should see the messaging centre number. Your best bet to solve the problem is to contact customer service for your mobile network. They can reset it.