Just call Sprint. They'll first tell you to dial ##"your MSL"# > Edit Mode and look at your MDN and MSID numbers to see if they match up to what they have on file. If they do, then they'll tell you to go back and dial ##3282# > Edit Mode > Enter your MSL > Data Profile > Menu Key > Restore. This will restore the data profile and go through the activation process again.
Most of the time the above will be the fix, but if it doesn't they'll tell you there must be an outage in your area, even though they don't see it reported, and then they'll do absolutely nothing, at least that's what they've done to me 5-10 times now. Data has been out of whack for me lately and I don't think I'm the only one.
If I've helped you, please hit thanks!