Send button in compose message does nothing

My mail.php's send button does nothing, when trying to send a user a message. I've restored the file from a fresh download, so that's not the problem. Where else should I be looking?

Well, I guess something had gotten foo'ed in the template files. I replaced the base template directory (thankfully, I don't edit those files) and it's all working again, so to anyone with the same problem, check your template files.