Have you Tried power cycling your M Give that a shot, and you might try running a System Cache Clear see if either gets your messages going again and make sure your Data is in Engaged b33
How big is the MMS file you are trying to send? Experiment by trying to send a really small file, say around 1mb just to see if it works. I have had MMS to fail when I tried to send a large file around 4mb. Hope that helps...